Plot hole: When Dan is chasing after Carter to catch him and Alex together at the restaurant, there's very little chance he could have had any idea where Carter was going. He couldn't have told the cab driver to "follow that taxi" because in the time between Carter getting in a cab and Dan getting in a cab about 5 taxis drive past, so how he happens to end up at the same place is a miracle.

Continuity: After Dan hits Carter he develops a bruise by his eye. It is there for the next several scenes but sometimes it looks red and swollen and sometimes it is quite faded then it will go back to swollen.

Deliberate "mistake": The scene where Topher Grace is jogging on his treadmill in his apartment is reversed. His wedding ring is on his right hand. In scenes before and after, his wedding ring was on his left hand.