Speaking of rules : I noticed the AI was able to castle even though one of the square in the path was attacked by one of my piece. I'm not a chess expert at all (this AI seems much better than me ^^) but I thought it shouldn't be possible.
I wouldn't be surprised if it doesn't cover that, I can imagine it'd be tricky to include without adding tens of bytes (10-20 bytes could be freed up by using slightly hackier function calls to/from the display, so perhaps Óscar will add more features/intelligence one day)
Was it a long castle? In that case, the squares the rook moves through don't block the move, it's only the squares the king moves through that block the castling move, so f8 and g8 for short castle and d8 and c8 for long castle.
In my experience having small webapps, it hardly helps. I literally added a huge modal popup on my feedback button specifically saying "DO NOT MESSAGE ME ABOUT X" before continuing and I still get dozens a week.
Even on reddit, when you get a high ranked comment, you will often get the same reply over and over again, even though a similar response is already the highest ranked reply to yours. There are people who just want to blurt out their opinion regardless of whether it's original or not..
Reddit sadly went the way of mostly write-only microblogging a long time ago now. I'm not sure what the solution is between invite only subs or heavy and time consuming moderation, and random comments that are just people thinking out loud, as using any sort of karma metrics would be useless when you can get 20-30k off a no effort post in a popular sub, but I also doubt that reddit the company spends much time worrying about it.
I can't remember who I stole "the internet is write only" but it's felt very true as audience size exploded.
Perhaps the worst part is that it's self reinforcing in either direction, and few people ever leave the agora unless they delve into niche subjects, so they bring the agora mentality with them.
I think this is the key point. As with most issues on the internet, it's mostly an issue of scale. I assume these problematic users are actually only a very small fraction overall, but as the overall size grows, so does the number of people who belong at the very edge of the bell curve in terms of "normal" behavior.
With scale, any rare issue becomes common enough to be annoying.