When the doctor tells the trio (Phil, Stu, and Alan) that Doug's blood work had come back, he tells them that Doug had a large amount of a drug called Ruphylin in his system. He then clarifies by saying "Ruphylin... roofies. Commonly known as the date-rape drug." Ruphylin is a euphemism for Rohypnol (whose makers would not want its name associated with the criminal activity depicted in the film), but a doctor looking at a blood work report would have likely referred to it by its generic name, flunitrazepam, or by its drug class, benzodiazepine.
When the 3 main characters and the baby go down to get their car (which subsequently is the police cruiser) from valet and are opening the door to get inside, the back door is opened for Alan and hits the baby, Stu says to Alan "you just nailed the baby" and clearly blames him for being the one to hit the baby with the car door, however, you can clearly see that Stu was actually the one who opened the door and hit the baby. Stu opened the door for Alan because he was using both hands to hold the sunglasses up to the baby's face.
In the opening scene where Phil is talking to Doug's fiancée, the scratch on his lip is perfectly visible. When they show that same scene later on in the movie, Phil's scratch is still visible, and Stu and Alan are still dirty from the day before. However, the night before, they had cleaned up for the blackjack scene, and none of their facial flaws were able to be seen.
On the way back to LA for the wedding. Doug is holding his hat with his left hand and reaches into his pocket with his right hand and holds up a bunch of chips from the Bellagio. In the next camera angle he is holding his hat with his right hand and holding up the chips in his left hand.
Alan admits he slipped a roofie into each of the shots taken on the roof of the casino. However, in that scene, he hands each of the other characters an empty shot glass and pours while they hold them. It would be extremely unlikely that none of the other guys wouldn't notice a pill in the glass already or Alan dropping one in each after pouring. The only possible explanation for this is that Alan put four roofies in the bottle of Jager before handing out the glasses, but doesn't seem likely since he says it was one hit each. Four hits in a full bottle would make individual shots more dilute than a full hit per shot.
When talking to Dr. Valsh in the hospital scene, Phil starts to write down "Best Little Chapel" on a piece of paper. He then asks Dr. Valsh where it is. After he tells Phil to get a map, the following shot of the piece of paper has nothing written on it and Phil rewrites "Best Little Chapel".
Towards the beginning of the film, after Doug says "it's not easy" and while Alan talks about Rain Man, a white van can be seen passing them quickly on the left (which Phil glances at). After Alan makes the mistake of saying "ri-tard" and Stu says "What?", the same exact white van is seen again, but this time going slower and moving backwards. In addition, after Doug corrects Alan by saying "REtard", the white van is no longer there.
During the taser scene both of Alan's arms are stretched out in front of him when the officer pats him on the rear. When the camera immediately cuts to the officer asking the kids about taking fingerprints, Alan's left arm is now by his side.
When Alan was hit with the taser and falls on the table, he initially falls on his side. When the angle changes to where Officer Franklin pats him on the rear, Alan is face-down on the table with both arms up. And when the children get up to leave the room, Alan's arms are down beside his side.
Alan has a plastic grocery bag with the liquor in the hotel room when he sings "Who Let The Dogs Out?" As they are walking down the hallway, he isn't carrying the bag. But it reappears when they are waiting for the elevator.
During all aerial shots of Las Vegas (between scenes), the Wynn hotel is being constructed. However, when the bachelor party are on the roof of the hotel before the night out, the Wynn is clearly visible in the background fully constructed.
Right after the scene where the guys let Mr. Chow out of the trunk. Alan was sitting against the front wheel of the car. As the camera angle changed he was sitting against the door. And as the camera angle changed once more, he was sitting in front of the wheel again.
When the guys are change their clothes on highway, Stu already got his suit and socks on, but en route when Phil straight his tie whilst driving and Doug shaving in the backseat, Stu is bare foot and just about put on his socks.
When Stu is sitting in the back seat with Little Carlos, he reaches over with his left hand to rub the baby's stomach. Carlos is on his right side, and Stu's right arm is straight down. When they widen the shot, Stu has his right arm resting across the top of the back seat, and then lowers it to his side.
When Mr. Chow comes out of the trunk, he beats Phil to the ground, which is dusty dirt. When Phil stands up later in the scene, after Mr. Chow has run off, his jeans are so covered with tan dust they look painted. In the same scene, after an immediate cut, the jeans have been brushed clean.
When the guys are in handcuffs at the police station, they are stretched out so that Phil can reach the pay phone on the wall. As he is talking, a cop wearing glasses is seen walking up behind him. The scene cuts away then back to Phil and the same cop is again walking up behind him.
In one of the wedding pictures, Jade is pointing to the ring on her left finger and her right hand is on top of her head. During the closing credits, the same picture is shown but her right hand has moved below her left hand now.
While driving to Vegas, the background continues to jump around to various spots on Interstate 210 in Inland Empire, California. Then quickly jumps to them driving on Interstate 15 past Barstow, Ca. (about 100 miles away). Similar occurrences happen on the way back from Vegas.
Towards the beginning of the film, when Alan makes the mistake of saying "ri-tard" instead of "retard", it appears that Doug is driving in the middle lane, as a car passes them on the left. However, in the long-shot where we see the car snaking up to the camera, it is obvious that it's only a two-line road.
While driving to Las Vegas we see that the guys are on the 210 west. To get to Vegas you then have to take the 15 north in the Rancho Cucamonga area. However there is a moment while on the 210 where you can briefly see a road sign for the Riverside Ave exit in San Bernardino. This means they overshot the 15 freeway by several miles.
When Alan is counting cards, he is quite visibly mouthing the numbers as the cards come by. While he was indeed eventually spotted by security, that was $80,000 and hours later. If there is ANY indication that someone is counting cards in a casino, they would immediately be escorted out.
Phil, Stu, and Alan are able to obtain the Mercedes from the impound. A car cannot be released from an impound without showing proof of ownership, which they did not have because the car was not theirs. However, while normally this is the case, the fact that they were released "off the books" provides a completely reasonable (or at least internally consistent) explanation for getting the car back without any red tape.
The formal name for "roofies" is given as "Ruphylin". The drug is really a trademark brand named Rohypnol, but was changed for the movie because the makers did not want to be associated with the depicted criminal activities.
Mr. Chow, at one point says "Suck on these little Chinese nuts", but speaks Korean to his "goons" when commanding them to remove the main characters from the car. Mr. Chow never calls himself Chinese, the previous statement could have been his attempt at humor (He is always making jokes and laughing.) Chow is a Chinese last name and it is entirely possible for someone that is Chinese to be able to speak Korean as well.
The Chinese violently ram the side of the guy's car with their SUV, but when they meet again the next day in the desert, there is no damage to the front end of the SUV. However, Mr. Chow is obviously a wealthy man and likely has more than one SUV.
Although the guys supposedly have no memory of the night before, they are still able to go back to Mike Tyson's house, even though he never gives them his address. However, it's not difficult to find out where a celebrity lives in Las Vegas.
The guys check in at Caesar's Palace on Friday afternoon for a one-night stay but are able to come and go to and from their trashed suite as they please, well into Saturday night. However, since they can still get into the room, it is obvious that they have extended their stay in the villa. This is highlighted when Phil tells Tracy that they are staying another night at the hotel to buy themselves more time to get Doug back.
After the scene where doctor tells them about the "Best Little Chapel", they are shown pulling up to the chapel in the patrol vehicle. When the step out of the car, Alan is able to get out of the backseat because patrol vehicles have child safety locks like many cars. A flip of a switch on each door allows them to be operable from the inside.
Doug, Alan, Phil, and Stu go to Vegas on Friday (evidenced by Phil saying "It's the weekend. You don't know me, and I don't know you" or something similar to one of his students after school lets out). The next day, presumably Saturday, they are hit with tasers as a demo to a group of schoolchildren on a field trip.
Stu is able to convince Melissa that he is wine touring out in the country instead of in Vegas. He says that the hotel that he's in has no phone or TV. If Melissa is "like a forensic accountant" (and therefore probably smart enough to see through a lie if there is a loose end), how would he explain the near-perfect cell phone reception?
Mike Tyson explains to the guys that he found out who they were and where they were because Doug left his coat in the tiger's cage, but later on when they are watching the security tapes at Mike Tyson's house, you can see Doug wearing his coat as they put the tiger in the back of the cop car.
In the scene where Chow jumps of the trunk with a crowbar and starts hitting Stu, Alan and Phil, the crowbar is prop and is obviously made out of rubber. Also when Chow hits Stu with the crowbar, the crowbar bends.
When they are preparing the steak for the tiger it is very soft and floppy, Allan is even able to fold it in half. However, when Stu takes the steak to be fed to the tiger it is very stiff, indicating that it is a prop.
In the scene where Phil, Stu, Alan and Carlos (Tyler) are at the hotel pool, Alan is jerking Carlos's hand to make it look like he's masturbating at the table. It is an entirely different baby in the next shot with Carlos.
When Eddy brings out the "Highroller Package" that was ordered, the picture of Stu and Jade is in broad daylight. Jade had said the wedding was at about "1". The valet ticket that Alan finds also says they were back at the hotel by 5:15am, before the sun would have even risen.
When Alan is sitting on the bench and the little boy walks by and takes a picture, you can clearly hear a picture being taken but you don't see Alan in the cell phone screen, clearly revealing its a prop.
When the main characters are attempting to get away from the thugs that are attacking them they back up and hit an Mitsubishi Eclipse they hit the car right in the quarter panel which would have obviously caused a huge dent, but as the Eclipse is being knocked out of the way you don't see any dents. Implying the car was pulled away as it was being pushed by the cop car with some kind of protective padding.
When Alan is getting up after the three guys are beat up by Leslie Chow, and Stu hits him in the head with the door, you can see Alan hit the car with his hand to make the thumping sound of his head hitting the car door.
The goof items below may give away important plot points.
After the guys find Doug on the roof, Doug tackles Phil. Doug immediately starts writhing in pain from his bad sunburn. Yet only a few hours later after the wedding, Doug chest bumps Phil and shows no pain.
Towards the end of the film when Doug finds $80,000 in his pocket, the exterior shot of the car shows the chips in his right hand, and his left hand is holding the strap on his hat, but on the interior shot, he has the chips in his right hand and the strap in his left.
When Phil calls the bride and says "we fucked up", there is major mistake taking place in the background. In the beginning the drug dealer Doug appears to be urinating throughout Phil and the bride's conversation, but later at the end of the movie when this scene appears once again, drug dealer Doug, urinates and returns to talk with Stu and Alan after we hear Phil saying "we fucked up".
Jade has bright pink nail polish on while she is dancing for Stu, but in the wedding photos at the end of the movie she isn't wearing nail polish and the wedding to Stu was supposed to be on her break from work.
When they find Doug on the roof, he is sitting under a ledge, and his body and part of his face is in sunlight. When Stu approaches Doug, however, his entire body and face is entirely in shade, indicating that the sun has moved between shots.
In the movie they went to the hospital after the wedding, because the doctor told them that they kept talking about this wedding they had been to, but at the end where they are looking through the pictures on the camera the hospital pictures are before the wedding pictures. However, as there is also a photo included of Doug passed out on his mattress halfway through the stream of photos, it suggests that the pictures are not actually in chronological order.
The digital camera that is found wedged 'in the backseat' of the car contains photos that explain many of their adventures over the night, including a few they had not been aware of, and are shown in the closing credits out of sequence. However, we see two sequences in the photo montage which occurred after they had dropped off the car, the visit to Mike Tyson's house (they were already in the police car by this point) and putting Doug on the roof (Doug was with them when they took the tiger, because he left his jacket behind and was seen on the Tyson security footage). Since the car had been abandoned before this point, these sequences should not have been on the camera.
During the wedding, the lady on the left is shown taking pictures of the flower girls, but the camera screen is never on, nor does she look through the view finder, showing that the camera was never on, and the sound was added later.