If a player sets a monster that is revealed to be invalid when it is flipped face up, what happens?

Question

Say that a player sets a monster. It sits there for a few turns and then the opponent attacks it, flipping it face up. Unfortunately, it turns out that it couldn't have been set. Perhaps it was a Link Monster (which can never be put in Defense Mode) or a Tribute Monster that was set without the proper tribute.

What happens? Given that a few turns have passed, it may be impossible to "rewind" to a point where the monster was set, so I'm not sure how the game can continue. In a tournament, would the player who illegally set the monster be required to forfeit the game?

Answer 1

In a tournament, the player who made the illegal play could be given a game loss, if the gamestate can't be reconstructed (and sometimes even if it can). Many calls like this are up to the discretion of the judge, and if you're at a larger tournament, you'll be able to appeal to a head judge for a final ruling if you disagree.

Be very careful about this. Even doing something like not activating a mandatory effect (like adding counters to a certain card when something happens) could give you a loss depending on the judge and if the opposing player calls a judge for it. I've heard of people being given game losses for not activating the effects of cards with multiple effects in the order printed on the card (Wavering Eyes in this case, for just the first 2 effects), even though the order wouldn't affect the gamestate in any way.

If you're playing with friends or in a more casual setting, oftentimes people just do the best they can to rewind it, or just play it out.

Answer 2

In a tournament, would the player who illegally set the monster be required to forfeit the game?

This would eventually be up to the Judge who tends the case, but most likely the offending player will surely get a penalty at least, or will lose that game.

Three things I think would happen here:

1. First, there is a chance no player ever realizes the mistake, in which case this would go unnoticed and the game proceeds "naturally". If any player finds out (of course, most likely the offending player won't), what should be done is to call a Judge so he/she can sort this out.

2. The Judge will then see if it is possible to replay or go back in turns to the moment where the illegal move happened. If it is possible then good, but if not the offending player will surely lose the game or receive some strong warning.

   To be honest, and if I were a Judge, I would indeed called out the game loss, even more if it were not able to "undo" it. I've seen players been given a lost game for less serious mistakes and situations, like showing up 30 seconds late to the table when you are up...

3. After the call is made, it should be resolved. If the Judge ruled for a game loss, the offending player loses, the game ends and the match continues. If this was a match-deciding game then the offending player will lose it. If another game is still to come, then the Judge would indicate so and both players would proceed with the remainder games.