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: 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: "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.