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)
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)