Revealing mistake: At Cafe Nervosa, after Niles has had a mini-breakdown, he has shed his clothes, boxers and all, all over the place and is supposedly naked in the corner with only a newspaper for privacy. When Frasier has him stand up to wrap aprons around him, one can see that David Hyde Pierce is actually wearing light-coloured briefs.
Match Game - S11-E18
Revealing mistake: 4 minutes into the show, Frasier gets off the elevator and goes to an office door with keys to open it, he tries the keys and they don't work. He realizes he missed his floor. A woman opens the door and they start talking about a dating service which he says he doesn't need, and he pushes the elevator down button, which lights up. The doors open and a man gets off. It's going up, which he finds out from an ex-girlfriend on the elevator. The down button light goes out before he gets on to go up with her. The light should only have gone off on the way back down.
Revealing mistake: During the scene where Frasier is dropping Charlotte off at the train station (the first time) you can see the background behind him shows that the car they are in is turning left. Yet we can see his hands on the top of the steering wheel and during this time he never turns the steering wheel.
Goodnight, Seattle (1) - S11-E23
Revealing mistake: You have to be very quick to spot this, but when the cannon goes off Niles drops the ring, you can actually see it drop between Eddie and the camera and bounce away from Eddie. In other words, it certainly doesn't go anywhere near the pate.
Chosen answer: The turkey's stuffing.