When Lloyd tackles Mary on the hotel stairs, she is wearing black pantyhose. By the time they get to the hotel room, she has mysteriously changed into gray tights.
At the beginning of the movie, Harry's van has bucket seats, yet later on when the "gas-man" rides along, he is sitting on a bench seat in-between Harry and Lloyd.
After shooting Harry, Nicholas' gun has the slide locked back, indicating that it is empty. In the next shot the slide is forward again.
Harry and Lloyd drive over the same bridge twice on their way to Aspen.
In the bar, Lloyd grabs the bartender with one hand but the next shot shows him grabbing him with the other hand.
When Mary's husband Bobby is getting out of the police car at the end of the movie he opens the door from the inside himself instead of having to be let out.
Rat poison doesn't kill people as fast as it killed the man.
Airbags do not remain inflated after they deploy.
Jet bridges have doors or barriers at the end precisely to stop people from falling to their death on the tarmac below when there is no plane to enter.
When Lloyd drives into the back of a car at the airport his air bag inflates and it looks it is made out of thin rubber translucent material. That is not what real air bags are made of.
On the note left for the "gas man" they misspell Aspen as "Aspin". This was most likely intentional since their lack of intelligence is a major part of the movie.
When Harry's legs catch fire, you can see the burn gel on his pants. While it is likely that the liquid on his pants is burn gel for safety reasons, in the context of the film, remember that Harry had just accidentally sprayed his legs with gasoline and the filmmakers were likely thinking that the audience would equate the gel with the gas.
Even though it is played for laughs the Chinese chef should had died instantly after Lloyd ripped his heart out as the heart is required to pump blood around the body. Plus the heart should not be pumping once it was ripped out. This takes place in a dream so realism goes out the window.
When Harry and Lloyd are reading the newspaper about the International Preservation Society, the exact same paragraphs are repeated many times throughout the paper.
During the tongue scene, when Mary is pulling Harry's tongue off the ski lift, you can see Harry's real tongue right under the fake stretchy one.
When Lloyd comes back for Harry on the scooter, it has two small training wheels on the back. They disappear in the montage of the ride to Aspen and reappear when they pull into town.
The snow in the camp fire scene is obviously fake (fabric and/or plastic).
The Aspen lift operators are wearing Copper Mountain uniforms. Copper Mountain is in Colorado, but it is over two hours away from Aspen.
When Harry is upset at Lloyd for driving the opposite way and is sitting on the shoulder of the interstate, you hear a bird singing. That bird is a Swainson's Thrush. Swainson's thrushes would not be located in this area as they are never found in this type of habitat. The bird sounds were most likely fake and dubbed in by producers.
When Harry throws the salt shaker over his right shoulder, Seabass's friends can be seen at the table in the background. They do not react after Seabass is heard being hit and shouting.
Immediately after Lloyd drops off Mary at the airport and starts to drive, he shifts the automatic column shifter only one notch (which is reverse,) yet the car moves forward.
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.
The Mexican hitchhiker's guitar playing doesn't match the playing that's heard. Also you can see him stop playing in the middle of the singalong while it audibly continues.
When Lloyd and Harry are in the bed crying, they are watching the last of a series of Pacific Bell commercials called "The Rain Children," but the dialog heard on the TV is from the first commercial in the series.
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.
When Harry is on the toilet at Mary's, the dolly track is visible on the floor.
When Harry and Lloyd are leaving Rhode Island in the van, a hand can be seen moving the curtain behind Harry's head.
When the doves appear in Lloyd's dream sequence, we see crewmembers releasing doves while lying on the floor behind two pairs of their own white sneakers.
When Lloyd asks Mary what are his chances of him and her being together, the camera crew is is reflected in the mirror in the background.
When Lloyd slides along the airport floor to pick up the briefcase, he stops at a strip of yellow tape which marks the place where he should stand up.
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.
The lift operators at Aspen are wearing Copper Mountain uniforms. Copper Mountain is in a different part of Colorado, over two hours from Aspen.
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.
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.
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.
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).
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 driving Mary to the airport Lloyd mistakenly thinks Aspen is in California. However later in the film it is implied he knows Aspen is in Colorado. It is likely either Harry corrected him where Aspen is or less likely realized he was wrong Aspen was in California.
When Harry And Lloyd get pulled over by the police officer, and they are caught with a open container, the cop drinks what he assumes to be a beer, yet this is today and at the time, illegal for a cop to do this. He could have faced potential charges including drinking on the job, and any charges he would have had on Harry and Loyld would have been dropped because this officer committed a crime. It's just something A officer wouldn't do.
Upon reading the "Dear Gas Man" note on the door, Mental concludes that Harry and Lloyd must have been following and evading him and his partner for weeks, yet he later hitches a ride with them without making the slightest effort to disguise himself.
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.
The police officer who pulled over Harry and Lloyd made an error in calling Pennsylvania a "state." Although they have the same status as a state, Pennsylvania is one of four commonwealths among the USA, including Kentucky, Massachusetts and Virginia.
Lloyd is shown laughing in a satisfied manner after Harry's toilet incident. The timing implies that everything that happened was part of his plan for revenge. However, he could not have known the toilet was broken, so to him the only cost to Harry was the, as it turned out, relatively short (20 seconds or so and not re-occurring) bout of diarrhea. Hardly great revenge for stealing the love of his life.