Monkeyman! / Buses, Bikes and Subways - S5-E1
Revealing mistake: Mr. Simmons counts 30 students in the bus, but nowhere near that many got off (nor have nearly that many fourth graders ever been shown).
Blue Wants to Play a Song Game - S1-E14
Revealing mistake: After Blue and Steve play the monkeys jumping on the bed game with the sock monkey, when the mail time song plays Steve gets up excitedly and his arm and hand get cut off.
Revealing mistake: When the Hulk is sitting on the edge of a building, Talbot suddenly appears in a helicopter to ambush him. If you watch closely, you can see the helicopter originates from behind Hulk instead of rising up from behind the building.