Continuity mistake: In the scene where Hal is confronting the two guys that made fun of Rosie he points to Rosie outside the restaurant and her shoes change from brown and black clogs (which is what she was wearing throughout the scene) to a plain black slide. Watch, it is totally obvious because they show her shoes several times before they suddenly change.
Continuity mistake: About the cake - not only the size of the slice changes, but the type of cake too. In the first shot, it appears to be vanilla on top, chocolate on the bottom. Second shot, it is all chocolate, and the third, it is chocolate on top and vanilla on the bottom.