Clash!! Sonic VS Knuckles - S1-E5
Revealing mistake: Right at the end of the episode, when Amy is telling Knuckles he should apologize, Amy has two right hands.
Revealing mistake: When Billy lies dead in the morgue and the President fixes his hair while saying, "That's better." we can see Billy's throat move as he swallows.
Revealing mistake: Reapers look 'different' in reflections and recordings, yet sometimes you can catch them with 'normal' reflections, eg. Rube in the diner window.
Revealing mistake: In brother Justin's padded cell, the far wall is obviously a painted backdrop. It even flutters when the orderly and doctor walk in! The only solid part is a couple of feet at the end, that Justin leans up against.
Lightspeed - S5-E8
Revealing mistake: When the HIVE Five exit the museum and are intercepted by Kid Flash, Mammoth loses both his pants and the artifacts he has before KF whizzes past him. It's best seen in slow motion; at normal speed this mistake is hard to catch.