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.
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 Topanga is "naked" under the cover, at one point, you can see a large corner of her bra. (00:02:58)
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.