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.
Star Crossed - S1-E6
Revealing mistake: About twenty minutes into the episode, Tru sits down next to Jen. Jen says, "I saw you in line in the cafeteria." Tru, with her hands at her sides, says, "Tru." Jen says, "Jen," and raises her hand to shake, at which point Tru reciprocates. But how did Jen know that Tru was introducing herself? After all, pretty much nobody else is named Tru, so in context it would have sounded like Tru was saying, "True," as in, "True; you did see me in the cafeteria."
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.