Other mistake: When Viper slides the swords to Jackie and they come to a rest next to him, there is a small paper spider right beside Jackie. The only paper spider in the room, however, is Origami himself, and he is actually too far away at that moment and too big to fit the bill. Guess some of the animators got their depth perception messed up.
Party Animal/Ipso Fatso/Don't Yank My Chain - S1-E2
Other mistake: [The correct episode is not listed] From Hero to Eternity: When Cuddles and Toothy are making the snowman and Toothy tries to put the hat on its head, in the beginning of the shot, there is a random smaller hat on the snowman's top button, which disappears mid-shot. (00:09:55)
Strategy X - S1-E1
Other mistake: When Nightcrawler pumps his fist in elation while standing in front of the mirror, his arm casts no reflection. (00:09:52)
Other mistake: In the episode where Tino is freaking out about the "FunVille" Arcade door looking like the mouth and face of a clown, it does. But in another episode I saw this morning (where Carver pretends to be all tough) the FunVille door is like any other normal door.