There’s something profound here about how the very natural conventions in Chess (“If you did that, you’d lose, so I’d rather we continue playing”) have redefined “valid” in a way that is surprisingly hard to re-implement with code.
https://www.chessgames.com/perl/chessgame?gid=2024861.
White castled short on move 10 and long on move 33. That didn’t help, though. White lost the game.
Also, it seems that site’s software doesn’t allow entering the full game.
Resolving irregularities does require your opponent to notice the irregularity and bring it to the attention of the referee.
Not sure about Blitz.
Note that it can be discovered during a game that a move several turns ago was actually illegal, and the board should be returned to that position (and times adjusted). I had this happen to me once.
Additionally, there are many contradictory rules that can't all be applied, so there is a lot of room for the tournament director to apply the rules according to the spirit of the game rather than the word of the law.
When I was in school, we used to play blitz like that. You lost track of your king, boom, you're dead
What is difficult is adapting chess's rules to this analog environment. It would be similarly difficult if you played this way with a real chess set in the real world, has nothing to do with code.