After Nick saves the girl, he has a cast on his right arm and hand. During the dinner/breakup scene, before dinner you see his cast under his sleeve. When the toast is made at the dinner, you see Nick's right hand and the cast is gone. Then when he breaks up with Kate, the cast is seen again.
In the diner when Kate offers to pay Nick to have dinner with her and pose as her fiancé, the knife in front of her keeps changing positions and at one time disappears completely.
When Nick comes to Man Ray to find Kate, his hair is relatively long, but when they leave his hair is short, and then long again in the next scene.
When Nick first arrives to see Kate at Mercer Advertising, she tells him to pick her up and carry her into the office. Just before he does so, he drops his bag on the floor to his left. In the next shot, the bag has disappeared.
After Nick and Kate's fake break-up, one of the wives throws a glass of water on Nick. The stain becomes larger and of a different shape between shots.
The ad the Kate Mosley comes up with for Gulden's mustard revolves around the fact that Cindy Crawford graduated 2nd in her class with a scientific degree. In truth, Cindy Crawford never graduated from college; she left during her freshman year to pursue her acting career.
When Kate is sitting in the seats, waiting to present to the group towards the end of the movie, right before she explains the lie about Nick, she orders food from a girl. When she goes to pay she finds the watch Nick left for her, but she never pays for the food.