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: The councillors put red paint in Beaumont's hat and put it on his head, squirting paint out through the top, but when they are pouring the paint into his hat you can see it already has a hole in it to let the paint out, and the way they are holding the hat shows them covering up the hole to stop the paint coming out before it is put on Beaumont's head.
Firefly, Light My Fire - S1-E1
Revealing mistake: Courage winces and reacts before the stereo-zapper is fired.