Bright Eyes - S1-E7
Other mistake: SPOILER: Gambit's funeral is taking place in his hometown of New Orleans, as evidenced by the coffins being above ground (because New Orleans is at or below sea level). However, Gambit's coffin is oddly lowered into the ground.
Deliberate mistake: When the news program is discussing Gyrich's assassination attempt on Xavier from the series finale of the original animated series, he's shown to have his classic look of auburn hair and sunglasses. However, when this scene occurred, Gyrich had black hair and was not wearing sunglasses. This was done deliberately to match the character design of the original series prior to the final 6 episodes, when Saban took over and redesigned much of the show's look.