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.
Continuity mistake: At the DoubleDoor Inn, where Rosie is seen dancing with the guy with spina bifida, we see the other guests applaud and approach them once they have finished dancing. And if you look closely you will see that Hal joins the crowd at the right hand side of your screen. But as it cuts to the next shot where Rosie is walking away from the crowd, we see Hal catch up with her from the opposite side where he was standing with the crowd.
Continuity mistake: In the first club scene, Jason Alexander is dancing with a swaying woman, behind them is a woman wearing a headband and sleeveless pink top; as the camera pans around the corner to Hal's new dance partner who is holding on to the railings, the same girl with a headband is dancing next to them, only this time with a different guy.