Continuity: The lighter on the rooftop moves on its own.
Continuity: In the first love scene in the movie Lisa takes her hair out of the clip it was in, then the scene angle changes and her hair's back up in the clip, when the angle changes again it's back down again.
Continuity: The wine glass moves in and out of Lisa's grubby hands.
Continuity: Lisa orders a pizza that is "half Canadian bacon with pineapple, half artichoke with pesto and light on the cheese." The half-eaten pizza seen later is clearly just a plain cheese pizza.
Plot holes: In an early scene, halfway through a conversation about planning a birthday party for Johnny, Claudette off-handedly tells Lisa: "I got the results of the test back. I definitely have breast cancer." The issue is never revisited during the rest of the film.
Plot holes: Chris-R's fate after the rooftop skirmish is also never revealed; however, an identical gun that which the drug dealer pulled on Denny later materializes in an open box laying on Johnny's bedroom floor.
Continuity: Early in the film, Lisa claims that she and Johnny have been together for five years. By the end of the movie (which cannot have lasted longer than a month, as the wedding never came), Johnny starts yelling about how they've been together for seven years
Errors made by characters (possibly deliberate errors by the filmmakers): When Denny runs in and discovers Johnny's dead body at the end of the movie, he screams "Tommy" instead of "Johnny" - Tommy Wiseau being the actor playing Johnny.
Revealing mistakes: When Johnny and Denny are talking on the roof and playing ball, one of the cuts make appear the ball is in the air for ages when they are only standing about 4 feet apart.
Revealing mistakes: When characters leave the roof via the stairs, they turn left. But the stairs physically can't go anywhere because of the layout. This is best highlighted when Mark leaves the roof.