Continuity mistake: When Fred says to Elizabeth "I'm going to have to be sick on you immediately, lie down!" you can see Fred grab the glass of water from the bedside table. Yet in the following wideshot, Fred jumps off of the bed without the glass of water, and he doesn't put it down anywhere, it's just disappeared. (00:15:40)Hamster
Continuity mistake: When Elizabeth bumps into Mickey, she drops her box of belongs, which scatter over the floor. She then puts them all back in the box, filling it right to the top. But when she gets up you can see halfway into box, and you can see there's nothing in it, but then when she's outside, sitting down, her box is once again full to the top with her belongings. (00:07:15 - 00:09:45)Hamster
Revealing mistake: When Fred gets his head stuck in the fridge you can see that his head isn't really stuck but merely bent down. Also, his hair goes in red but comes out brown.
Revealing mistake: In the scene where Drop Dead Fred and Elizabeth are pretending to be burglars you can see constantly throughout that young Elizabeth has fake long hair extensions in, this relates to later on in the film when Elizabeths hair gets cut off by Fred, but the fake hair extensions proves it was never that long to begin with.