Broken Link - S4-E26
Revealing mistake: During the fight between Garak and Worf near the end of the episode, a Jefferies tube door is knocked out of its frame to reveal the end of the set - a dark room with a wooden scaffold and various other pieces of equipment (slow-mo helpful but not required). (00:36:00)
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.
Food Fight - S1-E6
Revealing mistake: When Pudgy Pig is standing outside of the food packaging plant, a human hand can be seen sticking out of Pudgy Pig's wrist. (00:15:30)
Revealing mistake: While the computer is being infected with the virus, Peterson turns away from the screen to face Scully. Two seconds later, after the shot of Scully, the same shot is shown again but in reverse so that Peterson turns back to the screen. It's more obvious if you look at the weird way his eyes blink in the reversed shot. (00:39:50 - 00:41:32)
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)
Revealing mistake: "The Crash": Due to the use of both models and stock footage, what is supposed to be one airplane has three very different appearances. As the passengers board, we see the "real" plane. In the air, it's another plane altogether (stock footage). When it crashes (a model), it looks completely different yet again.
Dr. Quinn, Medicine Woman - S1-E1
Revealing mistake: While Dr. Mike is riding in the wagon with Charlotte and her children, when Charlotte says, "I'd like to ring Sully's neck," a vehicle is visible behind Charlotte's head driving toward the left side of the screen. (00:20:05)
Revealing mistake: About 20 minutes in, the cops have a newspaper with the headline about the two guys they killed. They turn to page two which has a story about the next guy they are planning to kill. If you look at the picture, it is glued onto the story page. The left side is lifted. The actor must have been hired after the printing.