Continuity mistake: When Harry and Mary go out on the date to the ski resort, they get into a snowball fight. When Harry is shoving her face in the snow, the first time it shows Mary she is facing one way, but when they show her again she is facing a different way. In addition to this before the camera cuts to a different angle on Harry you can see Mary's nose is bleeding immediately after she stands up. The camera then cuts to a different angle and it isn't. (01:18:21)
Continuity mistake: When Mary leaves her briefcase on the airport floor, Jim Carrey is seen running and sliding to the briefcase. In the scene directly preceding that, he is nowhere to be seen, yet instantly he is teleported within 20 feet of the briefcase. (00:09:35)
Continuity mistake: The film appears to begin in the summer. Look at the outside shots in Rhode Island when either Harry or Lloyd are driving; green grass and many leaves on the trees. As they travel further west the landscape appears to be fall. Then finally in Colorado it appears to be the dead of winter.
Factual error: When Lloyd drops Mary off at the airport, he watches her leave and then hits a parked car, inflating the airbag. First, the Cadillac Brougham Lloyd is driving was never available with a driver-side airbag, and secondly airbags are not designed to inflate after such a low-speed impact.
Audio problem: Just before Lloyd and Harry set out for Aspen on the mini bike, they clumsily attempt a high five. At this point Lloyd asks Harry if he still wants to go to Aspen, but when he says, "Oh yeah" his lips are forming altogether different words. (00:55:00)
Other mistake: In the scene where Lloyd hears footsteps approaching his bathroom stall, he locks the door so that Sea Bass can't enter without forcing it open, as seen in the following shots. But if you look closely at the space between the partitions as Lloyd turns the locking handle, you can see how the bolt moves only halfway between the door and frame thus never actually locking. (00:48:05)
Continuity mistake: Right after Lloyd runs into the jetway at the airport, the guy who witnesses his fall moves to the window to watch. He is grinning from ear to ear in his reflection, but immediately has a 'that had to hurt' look in the next shot. (00:10:15)
Continuity mistake: When Lloyd is driving Mary to the airport, at one point he says "What's the matter, nervous about the flight?" When he says this look at the white van in the rear window, it's right up next to the limo. However when the shot cuts to Mary it's already several meters behind them despite the very slow movement the limo has made. (00:04:30)
Continuity mistake: When Lloyd tells Harry to throw some salt over his right shoulder, he puts the salt shaker next to Harry's burger and fries. The next shot shows the salt shaker behind the basket of Harry's food. (00:22:49)
Continuity mistake: When Sea Bass demands to know who hit him with a salt shaker in the diner, we cut back to Harry and Lloyd's table and Lloyd begins pointing at Harry. When the camera angle changes, Harry turns around and Lloyd begins pointing a second time.
Continuity mistake: When Harry delivers the dogs, there is no carpet on the floor in the car. But when the indians are sitting in the car suddenly there is a carpet on the floor. (00:38:55)
Other mistake: On the subtitles for the UK "Uncut" DVD (I'm not sure if this applies to other "Unrated/Uncut" DVDs), the subtitles mix up the character names during one shot. As Lloyd goes in the wrong direction, the film cuts to a shot from behind the pair. Harry says that he "expected the Rocky mountains to be a little rockier than this." The subtitles incorrectly state that this line was said by Lloyd. Lloyd's response that "John Denver is full of shit" is also incorrectly attributed to the wrong character; the subtitles say Harry said that.
Answer: It was a commercial for Pacific Bell. A telephone company.