Continuity mistake: In Gambit's flashback it is shown that the chosen one from the Guild of Thieves is banished to a "darkness never-ending" for breaking the covenant of the tithe. However, the same character can be seen later in the episode fighting alongside his fellow guild members.
Night of the Sentinels: Part 1 - S1-E1
Other mistake: In the scene where Professor X uses his psychic powers to deduce that Jubilee is trying to escape the mansion, there is a semi-dissolve edit that simultaneously shows a close-up of Professor X with his left hand on his forehead and a counterpart image of Jubilee hiding around the corner. If you look closely at Professor X in the counterpart image of Jubilee, his left arm and hand are shown to be on his hover-chair's armrest and not his forehead.

The Phoenix Saga (1): Sacrifice - S3-E3
Revealing mistake: After the X-Men rescue Dr. Corbeau and retreat back to the shuttle, they come upon a room with crew members passed out on the floor. Keep an eye on Wolverine in the background on the right side of the screen and you will see him literally walk through a door panel. (00:17:28)
Audio problem: Cable, Erik the Red and Captain America all have the exact same voice.
Chosen answer: It says MPO "First Exemplary Printing House" which is a real printing company in Russia founded by Ivan Dmitrievich Sytin in 1889. There seems to be something about "named after..." as well.
Sierra1 ★