Revealing mistake: In "Not Normal" when SpongeBob says "Hmm. This video tape should help," his left arm is missing. (00:01:25)
From Method to Madness - S3-E18
Revealing mistake: After the fight between Stewie and Olivia at the concert hall (followed by their breakup), Simon tells Stewie that the act without Olivia is "over" as he turns to walk off the screen. However, before Simon is even partially out of the shot, he completely disappears. (00:18:45)
A Bicyclops Built for Two - S2-E13
Revealing mistake: At the end of the wooing near the end, you can see that Fry has four arms.
How The Elephant Got Its Trunk - S3-E6
Revealing mistake: The white around Papa's lips and eyes is slightly running when he is sprayed with water. however the white is supposed to be part of his real skin.
Stalk Like An Egyptian - S1-E10
Revealing mistake: At the beginning when the Egyptian mummy puts the ring on his finger, the tip of the finger bends like a glove.