I oversaw a club tournament for a grade school club, k-5, a couple of times, and some weird stuff happened. This was an unrated tournament and the kids were not "serious" players. (My son won the fourth grade tourney rather easily at a time when his USCF rating was about 500).
I was watching a game that began with 1. e4 e5. 2 Pawn on e4-d5 x e5 (?!?!?) I stopped and asked what that was and she explained, "It's called en passant." I explained that no, that wasn't really en passant, but she was mightily insistent on it, and was quite irritated when I insist she take back the move. I also started to demonstrate the real en passant, but she was rolling her eyes and making it very clear that she was annoyed by this bothersome grownup who thought to lecture her on the rules. Her opponent had no objection to the move. She made another move and I turned and walked away. As I turned back to look at the game, she made the same move she had tried before. This time, I decided that if her opponent didn't know any better, and was letting her do it even after I explained it, there wasn't much point in intervening.
In a game between first graders, the kids were required to have one of the adult directors verify each checkmate, as they frequently missed moves that could get them out of check, especially if those moves required interposing. I was called over to verify a checkmate. The white king was on A1, the black queen was on A5. Meanwhile, there was a black pawn on C3, guarding the B2 square, which was occupied by the black king. (????)
I wasn't quite sure what to do about that one. I tried to explain that maybe there was an illegal move in there, but as black had several pieces and white only had his king, we could probably call it a win for black.

And I really like the way Firefalcon argues it. If you can capture to stop the mate, then it isn't a mate in any other situation, so why would it be one if the capture is en-passant?