This Male Chauvinist Piggy Went to Market - S3-E1
Revealing mistake: When Laurie goes to flip Goose over her shoulder, it's obvious that a stuntman is actually flipped; he has much shorter hair than the actor portraying Goose. (00:11:45)
Revealing mistake: During the show when doughnut dressed as a tramp sits on the end of the bench and his weight causes tiger, who is sitting on the other end of the bench, to go up in the air, you can see the wires lifting her.
Firefly, Light My Fire - S1-E1
Revealing mistake: Courage winces and reacts before the stereo-zapper is fired.