When Lloyd is running up to the screens that tells the different flights on it, while he tries to find the flight for Aspen while saying eeny, meeny, miny, mo, he says "mo," but his lips don't move when he says it.
When Harry and Lloyd are at home, Harry opens a beer and throws the bottle cap on the floor. It makes the sound of hitting a hardwood/tile floor even though all shots of the living room floor show it to be wall to wall carpet.
After the dinner scam, Lloyd tells Harry about the movie he saw. When he mentions the guy getting his throat slit, you can hear Harry speeding the car up, however the background outside does not move any faster.
The officers at the scene of Mental's death report that Harry and Lloyd were last seen headed west on I-80 towards Colorado. I-80 does not go through Colorado at any point; it passes north of the state through Nebraska and Wyoming.
When Lloyd states that the map says they've only driven about four inches, there's a creek with an unusual tree growing around it. We see the same creek and tree when Harry and Lloyd pick up the "gas man."
When Lloyd drops Mary off at the airport and hugs her goodbye, in the background there are two men walking towards them. The camera changes to the angle behind Mary then behind Lloyd and then back to the side. When it returns to the side view the two men are farther away but still walking toward them.
When Harry has his tongue stuck to the chairlift, there is frost on the pole when he licks it. Later, when he asks the kids for a cup of warm water, there is none. Then when Mary removes his tongue, the frost is back.
When Lloyd sees Mary's photo in the newspaper, he struggles to read the headline; he cannot even decipher the word "the" without Harry's help. However, in other scenes he can read perfectly (such as when he reads the graffiti in the men's room or the newspaper clipping in the bar).
When Lloyd is in the gas station bathroom and first sees the "For Manly Love" writing on the wall, it says "BE HERE MARCH 25TH AT 2:15AM SHARP". All other shots of the writing say "MEET ME HERE ON MARCH 28 AT 2:15AM SHARP!".
The Cadillac limo Lloyd is driving at the beginning is from the early 1980s and would have no airbag. The steering wheel is obviously not equipped with one in the shots either. After Lloyd drops off Mary at the airport, he gets back in the car and the wheel has been swapped with an airbag-style one for the scene where it deploys (with an inflated garbage bag of some sort).
When Harry and Lloyd first arrive at the truck stop, a man with a white hat leaves the diner after the eighteen wheeler passes by. After Sea-Bass spits in Harry's burger, the same man is about to exit again
When Lloyd is sleeping on Harry's shoulder in the van, it seems his head on the left side is leaning on Harry and messes up his hair. When they stop at the gas station, Lloyd's hair is messed up on the right side when he is in the bathroom.
When Harry and Lloyd are in their hotel rooming talking about meeting Mary at 10PM instead of AM, Harry starts walking out of the room. He stops and turns to look at Lloyd and says, "10AM..". As he says this there is clearly a wall right next to him. When the picture pans out to show Lloyd and Harry, there is no wall remotely close to Harry.
In the opening sequence, the limo Lloyd is driving appears to be around 6 to 8 feet away from the girl on the sidewalk. When the camera angle changes and Lloyd talks to the girl, the limo appears to be only 2 feet away from the sidewalk.
When Harry and Lloyd both get back from work and they are sitting down, a glass bottle is standing up inside the small end table between the couch and the chair. Later in the scene, the bottle is knocked over.
The time of Harry and Lloyd's trip takes place in late March, yet as they pass through Pennsylvania many of the deciduous trees have green leaves. At that latitude trees actually don't start sprouting leaves until late April.
When first driving out of Rhode Island, they are heading north on Route 146, because you can see the Union Paper building in the shot. In the next shot showing the highway as they are driving, they are on 95 South, passing the "Big Blue Bug", a local pest control company building. There is no way they can be driving on both of these highways at the same time, they head in opposite directions.
In the scene where Harry and Lloyd are on the bed in the presidential suite, crying and blowing their noses with money, they are watching a commercial for Pacific Bell. If they are in a hotel in Aspen, CO there is no way they would have seen that commercial on a hotel TV when the ad is clearly for a company that serves the west coast.
In the second part of the movie Harry and Lloyd are supposed to be in Aspen. However, that part of the movie was filmed in Breckenridge. The town of Breckenridge has one main street where shops and restaurants are located with streets descending hills feeding into the main drag. Aspen is a town spread out across many blocks with rather busy traffic. This information was confirmed by the public relations director for the Breckenridge ski area.
After Harry and Lloyd flee Dante's Inferno, an FBI agent states that "they were seen heading west on I-80." The next road you see them on is a rural two lane highway. Not a big four lane interstate like I-80.
Just near the end of the film Lloyd and Harry are seen in a desert presumably the Phoenix Desert just days after Mary reunites with her husband. Given that the Phoenix Desert is many miles from Colorado it would take weeks for the duo to get there on foot.
When Harry, Lloyd and Mentalino stop to eat, the sign identifies the stop as the "hottest meals east of the Mississipi." After Harry and Lloyd leave, it is established they are in Iowa, which is west of the Mississipi.
After the FBI rescue Lloyd, Harry and Mary from Nicholas Andre, Beth tells Lloyd and Harry they have been following them from Providence. However Beth and her colleagues were not seen in the film until after Joe was killed and didn't hear of Lloyd and Harry until they escaped.
When Harry and Lloyd stop to pick up Mental at the side of the road, he claims that he's late for a luncheon. However, he then proceeds to stop for lunch with Harry and Lloyd (although it's possible that he assumed by their behavior that they were too stupid to realize this).
Mental ends up with Harry and Lloyd by acting stranded. He seemed very confident that they would stop. He also has the hood up in the car and tells his partner to lie in the don't seat. Not only would she not see them leave, but she would have also had to put the hood down to follow him. There is also the question of where she was when Mental dies in the restaurant.
When Harry and Mary are making a snowman, there is pair of skis stuck in the snow with the top hat hanging on them. But they have both changed out of their ski clothes, neither of them are wearing ski boots, and there is no other ski equipment there.
Outside the airport, after bumping his limo with another car, the airbag inflating towards Lloyd's face correctly originates from the center of the steering wheel. In next scene, while he is fighting with it in order to gain access to the airport, it has been replaced by another kind of a balloon, clearly not coming out from the steering wheel.
The goof items below may give away important plot points.
Harry and Mary spend the early part of the day together and make plans to meet the evening of the same day at 7:45 pm. Later on that night (same day) as the three (Harry, Lloyd and Mary) are being held at gunpoint, Harry says, "Yesterday was the best day of my life. Mary and I went skiing....etc". But it wasn't 'yesterday', it was earlier the same day that Harry and Mary went skiing.
After Nicholas tries to stop Harry and Lloyd from entering the ball and Lloyd sprays Binaca in his eyes, J.P. Shay stands next to him and says that they are the guys who "whacked Mental". The next scene Nicholas is on the stage with the snow owls, but then is instantly off stage and back at the entrance of the room with Shay again.
After Nicholas shoots Harry and is about to shoot Lloyd, his gun is obviously out of bullets as the slide part of the gun is cocked back and the barrel is exposed. Then in the very next shot, the gun is loaded again.
Harry throws the salt at Seabass in a Pennsylvania diner. Later the same man attacks Lloyd in a Nebraska bathroom. It's possible that Seabass is a trucker and happens to be in different parts of the country at different times through traveling (the diner and gas station bathroom would not be unfamiliar places for truckers).