Continuity mistake: When Joy goes to work after moving in with Jack, she is in a blue dress. When she is at work, her dress is grey - the same one she had on at work at the beginning of the movie, before the break up. But when they meet later at the counselor's, she is back in blue. (Not to be confused with the red jacket she is wearing while she is working.)
Continuity mistake: When the bald "guy" hits Ashton Kutcher in the face with a hard right, you can see that the fist hits Ashton's left eye and his face turns to the right. But in the scene after, he comes in with the "blue-eye" and it's on the right eye, the eye that didn't get the hit at all.