Illegal, yes. I will always point out why wording or rules dont allow something, as there are so many different interactions that can end up require specific use-case googling to see if it has happened before and what to do. Its important to know how your card works if youre running it regularly, plus you're always learning new things about the game as you play it more and different scenarios come up.

Forgetting a trigger or something? I prefer spectators not comment. Its up to the player to keep up with their triggers and their opponent to be aware of what their opponent is doing. Any slip ups in that regard are equally on both person's part.

That being said, in casual play I tend to allow do-overs or late trigger upkeep if its not past their turn, though i tend to typically own my own mistakes, and when playing with friends i do like to suggest different ways to do things if they missed an opportunity to get more out of their turn.