Especially in blitz or bullet games, it is possible that a player makes an illegal move, for example castles through check. 

* If the opponent does notice the illegal move, then as far as I know the first player is obliged to make a legal move with the same piece, if one exists. What if there are no legal moves? 
* What if the opponent, being in time pressure, doesn't notice the illegal move and makes a move. What happens after they realize that? Does the person who made the illegal move forfeit? Or is the whole game cancelled?

Are there any standard rules for these kinds of situations?
## Answer (1)

Here's my take from USCF blitz tournament rules, USCF bullet tournament rules and some internet consensus:

> If a player makes an illegal move, his clock shall NOT stop running unless a TD or a senior arbiter present determines that the game should end.
>
> In blitz games, an opponent is required to point this out to the player within one (1) minute of the occurrence, or the illegal
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Comment: These are rules for tournaments. For casual games, the rules are almost certainly defined by the agreement between all the players. In that context, "what happens" would be a question about player behavior rather than the rules.

## Answer (2)