When Mike and Phoebe are talking about getting rid of the baby rats, Phoebe puts the box on the coffee table. In a subsequent reaction shot of Mike, the box is back in her lap.
When Rachel and Gavin are in their office trading insults ("Oh, they didn't tell me about your quick wit!"), Gavin alternates between sitting on the edge of the desk and standing in front of it depending on the camera angle.
At Rachel's party, Ross asks Chandler where Molly and Joey are and Chandler puts the balloon to his lips to take a breath from it, but there is a quick edit and Chandler says, "Right behind you, big guy." There isn't enough time for Chandler to have taken the breath and the balloon is nowhere near Chandler's mouth as they start walking out of the apartment.
When Rachel and Gavin are talking in Rachel's office, the secretary Heather walks in to drop off something. Rachel calls out Gavin for "checking out" Heather. Gavin says "I'm in fashion and I was checking out her skirt", but Heather was obviously wearing pants.
Actually the line was "I'm in fashion, I was looking at her skirt...or was it pants? I didn't really see what happened below the ass area", so basically the whole skirt/pants situation was meant to show that he indeed was checking her out disregarding the clothing.
Mike sets the rat trap and puts it on top of the refrigerator, but when he goes to remove it the trap isn't triggered.
Ross leaves the party to chase Joey down to get him away from Molly. Shortly after Ross gets to Joey and Molly, Molly's girlfriend arrives, and they leave like Molly was scheduled to be off work, but Ross was still supposed to be at the party, so she should have still been working for a while longer.
When Rachel is talking to Monica in her office about not inviting her horrible work colleague to her birthday party, we see the name plate on the office door written as Rachel Greene....Rachel, however spells Green without an "e" on the end.