Revealing mistake: When morphing, a "Net" appears over the Rangers, which their Zeo powers then morph them over. Tommy and Tanya's nets don't match their bodies - for example, their shoulders aren't in the net, yet they disappear when their Ranger costume appears.
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Revealing mistake: Timberly Loves Arnold: When Lila says, "That is, if Arnold doesn't mind making it a threesome," her left eyelashes disappear for one frame. (00:05:50)
Revealing mistake: When the Gamma Warriors are running towards Hulk on the rooftop, you can see two creases going vertically across the screen, likely from the edges of the animation cells. (00:20:18)