Revealing mistake: There are numerous times throughout the series where the supposed big city of Metropolis is actually just a back lot set and none of the buildings are more than a couple of storeys high. Quite often the green hills of Southern California can be seen beyond the set and between the buildings.
To Thine Old Self Be True - S7-E20
Revealing mistake: When Reagan drops her bag in the hallway when she is talking to Frasier, a lettuce rolls towards him. It didn't fall from her bag: if you look closely you can see it was tossed into the frame from just outside the camera view, obviously to ensure that it would land at Frasier's feet for the comic effect of him kicking it back to her.
Revealing mistake: The stick of dynamite Brisco throws lands to the right of where the pyrotechnic for the dynamite explodes.
Revealing mistake: When Fran is on the private jet, the champagne sitting between her and Cathy doesn't move through all the turbulence, showing that it is the camera shaking and not the plane.
Revealing mistake: When Topanga is "naked" under the cover, at one point, you can see a large corner of her bra. (00:02:58)