Deliberate mistake: There's at least two episodes (Season 1's "Captive Hearts" and Season 4's "Sanctuary, Part 1") in which a character shows video footage of the X-Men and various supporting characters in action, but the footage is merely clips from previous episodes.
Night of the Sentinels: Part 1 - S1-E1
Deliberate mistake: When the Sentinel is first walking through the mall, its footsteps are so heavy that it shakes the building. When it is shown walking past the shop Gambit is in, there is no shaking of the building, presumably done so Gambit wouldn't notice it. (00:04:08)
Deliberate mistake: The video footage Callisto has of the X-Men battling Sentinels at the Mutant Registration Center is just clips from that episode. (00:09:27)
Days of Future Past: Part 2 - S1-E12
Deliberate mistake: When the crowd is protesting outside the Capitol, the same man with black hair and a brown outfit has been drawn numerous times in the shot. (00:10:23)
'Til Death Do Us Part: Part 1 - S2-E1
Deliberate mistake: When the Friends of Humanity are secretly filming their fight with Beast, Wolverine and Gambit, their cameraman is shown to be filming from the street behind a post box. However, the footage that they submitted to the news is merely the same camera angles shown from the viewer's perspective of the show. (00:11:45 - 00:13:20)
Deliberate mistake: When Gambit and Pierre are boating down the river, in the shot of Pierre saying "Yeah, we got word," the trees they pass on either side of the screen are just the same tree drawn repeatedly. (00:08:08)
Mojovision - S2-E11
Deliberate mistake: Some of the footage of the X-Men in action that Major Domo shows Mojo is clips from previous episodes, and even clips from the show's opening theme. (00:03:36 - 00:04:22)
Deliberate mistake: The security footage of Proteus' escape from the Muir Island Research Facility is merely the same earlier scene played over again. (00:06:50)
The Lotus and the Steel - S4-E13
Deliberate mistake: Every native of the Japanese village that Wolverine visits is conveniently fluent in English, and they even speak English to each other when no other people from English-speaking countries are around. (00:04:45 - 00:20:45)
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 ★