Revealing mistake: Will is reading a book in the total nude on the sofa, and then Leo walks in on him. When Will gets up from the sofa to run into the bathroom, you can blatantly see pants around his waist as he gets up from the sofa and runs to the bathroom.
...And the Horse He Rode in On - S5-E1
Revealing mistake: At the beginning of the episode, Grace is running through the park, and she unexpectedly runs right into a lamppost. When she hits the lamppost, the lamppost shakes very violently, like a cheap set lamppost.
Revealing mistake: When Grace plays on the Dance Dance Revolution machine in the arcade, at the bottom of the game screen it reveals that it is a demo, and also says "Free Play."
Revealing mistake: When Grace steps on the Dance Dance Revolution machine, she positions herself in a way it looks like she's playing on Double Mode, but the on-screen arrows in the game clearly show that she's playing on Single Mode.
Answer: The director, James Burrows.