Boys in the Band / Hex Games - S2-E2
Deliberate mistake: When AJ and Chester show Timmy "The Timmy Tuck", AJ says it has a 1400 degree turn. The diagram shows 140°.
Chin Up! / Dog's Day Afternoon! - S1-E5
Deliberate mistake: When Cleft (Timmy) and the Chin go to the last page with the letters and they get to Timmy's letter, Chin says he spelled Chin wrong yet you must see it's spelled right.
Mighty Mom and Dyno Dad / Knighty Knight - S2-E9
Deliberate mistake: Timmy's eyebrows show through his armor.