Revealing mistake: Season 2, episode 4 (Duets): When Brittany lifts Artie from his chair and carries him to her bed, right before she puts him down, he is obviously lifting his feet to prevent them from bumping into the bedframe.
Revealing mistake: At the very end Vince is DJing with the cobra, and when the cobra starts dancing you can see the fishing wire attached to the snake to make it move.
Revealing mistake: During "Brand New Day", watch the background as Dr. Horrible appears as a giant and stomps down the city-street. As he takes his first step, a piece of concrete from building to his right falls off and hits the ground. However, if you look closely, it's obviously a quickly-made computer effect. As the piece of concrete hits the ground, it simply vanishes, and is replaced by a rather odd-looking animated "poof" of dust. The issue is the concrete piece was much bigger than the cloud of dust, so it's clear that the piece simply disappears right as the dust-poof appears.
Revealing mistake: When Mr. Goodman blows his nose, you can see a little pink from the makeup on his nose (to make him look sick).
Monster in the Playhouse - S1-E9
Revealing mistake: As Reba the Mail Person exits the Playhouse, she goes past the window, and you can see her exit the sound stage through a door painted the same as the background.