Suppose that a game play error occurs during a tournament. Both players agree that they would rather handle the error themselves than call a judge, and so they agree on a resolution and continue to play.
- Sarah accidentally looks at the top two cards of her library during her draw step. She asks her opponent, "Is it okay if I just take the top card, and shuffle the other card back into my library?" Her opponent says that this is okay, and so Sarah does this, and both players continue.
- During combat, José mistakenly believes one of his creatures has taken lethal damage, and puts it into his graveyard. His opponent doesn't notice the mistake. A couple of turns later, José's opponent realizes José's mistake and points it out. José says, "It's okay, we'll just keep playing", and his opponent agrees.
Is this legal, or is it an infraction of some kind (perhaps collusion or cheating)?