Harold insists on going to the ER to find out if he has rabies. You cannot find out that soon after an attack, and usually you have to take the potentially rabid animal in with you, because they dissect the brain to determine if an animal is rabid.
When Harold and Kumar are hiding in the girls bathroom and peeking out over the top of the stall, the feet of one of the sisters can be seen in the stall next to theirs before she is actually shown going in.
When the extreme punks are in the store, and Neil Patrick Harris steals Harold's car. Harold and Kumar look around the parking lot, the extreme Ford truck is missing, even though the punks are still in the store.
When Kumar was on his cell at the start of the movie and he shocks the principal, the guy drops his cup of coffee and it spills on him. However, in the next scene, it appears back on his table. After Kumar is done and when the guy starts to talk, we can see that the cup is nowhere to be seen on his desk.
Harold's car (2003 Toyota Camry) has alloy wheels, yet when they pull over for Kumar to urinate in the woods, the car has cheaper hubcaps. When the raccoon climbs in you see a close-up of the alloy wheels again, then again see hubcaps as the car pulls away. Not only that but Harold's car has leather seats, throughout the film yet at the shot of the drive through at burger barn, you can see that the car has cloth seats.
When the Burger Shack guys goes insane and yells, "Let's burn this mother down," he loses his hat while knocking things down. But when he rises to talk to Harold and Kumar, his hat is back on his head.
When Harold starts yelling about having no money at White Castle, he bangs down on the counter knocking over the yellow mustard container. The mustard is alternately standing/lying down in subsequent shots.
When Harold and Kumar escape from the cops in Princeton, Kim, the twins and the "I'm so high kid" are in a corridor that is well lit. When the camera moves over to the other side, that corridor is empty and dark.
In Kumar's Med school interview, his collar alternates position. He begins tucked with tabs in, then one side tucked under his jacket, then both tabs out, then the opposite tab tucked, completely tucked in, then both out.
When Harold and Kumar are about to eat the burgers, Kumar stops Harold from eating his burger so they can bite into them simultaneously. Prior to this, Harold's burger is still in a box and suddenly it's out of the box.
When Harold and Kumar almost crash into Goldstein and Rosenberg, when the two approach the car, Kumar rolls the window down all the way. Then in the next shot where you see Kumar get out of the car the window is slightly up.
When Harold and Kumar almost run off the cliff in the punks' SUV, the front left wheel is hanging off the cliff in mid-air. A few seconds later, as they hear the siren and Harold says, "We're trapped!", the wheel is plainly seen on solid ground.
When Harold and Kumar are arguing outside the gas station, a sign that says "Self Server" can be seen in the background of the shot by the pumps. New Jersey does not allow self service at gas stations, only full service.
While the geography of New Jersey in the movie is very accurate during Harold and Kumar's journey from north Jersey to south Jersey, there is simply no need for them to have driven so far to find a White Castle. The trip from Hoboken to Cherry Hill is 85 miles and takes about 2 hours (without traffic), and there about 20 White Castles along the way. In fact, Harold and Kumar didn't even have to get on a highway to find one; there is a White Castle on Kennedy Boulevard in Jersey City, which is adjacent to Hoboken.
Contrary to the plot of the movie, there is no White Castle located in Cherry Hill, New Jersey. When the film was written, there was a White Castle in Pennsauken, NJ (a nearby town). Currently, the nearest White Castle is in Jersey City, New Jersey, which is actually about 15 minutes from where the movie begins. The nearest White Castle to Cherry Hill is in Toms River, NJ.
At the gas station, a "Country Style" donuts sign is visible on the outside. This is a Canadian chain of donut stores which would not be in New York/New Jersey. Other Canadian stores such as Shoppers Drug Mart and Chapters (a bookstore) can also be seen.
When Harold and Kumar stop the Bronco just in front of the cliff, only the left front wheel is hanging over the cliff. When they try to back up, that wheel rolls freely as if they can't back up. That front wheel rolling clearly shows the vehicle is in 4 wheel drive, and even if it wasn't, all Ford trucks are rear wheel drive by default, meaning with three wheels still on solid ground, the Bronco should have backed up with ease.
The slider special (as shown on the TV commercial at the beginning) is $2.99 for 6 burgers, fries and a drink. If they ordered 5 specials each, their bill should be just under $30 before tax, and yet they are charged almost $50. However, in the finale they order separate items (30 burgers, 5 fries, and 4 drinks each). Unless the server converted that order into specials (8 specials, plus 10 burgers and 2 fries), they would not get the cheaper price.