So I had an interesting thing happen while playing a friendly match (no event). My opponent has 4 mountains. He cast a Risk Factor during his main phase, I chose to let him draw the cards. He now has 1 mountain untapped and uses it to pay for the spectacle cost on Light up the Stage, and he promptly flips over the top 2 cards of his library, but UH OH, he never did damage to me this turn and he doesn't have enough mana to pay for it. What would be the best fix for something like this should it happen at FNM? For our game I chose to just have him shuffle the 2 cards back into the deck and return the Light Up the Stage to his hand.

I would also recommend that he untap the Mountain he tapped to attempt to pay for the spell. I see no reason to not do this fix, unless somehow those cards were known before hand (which I find extremely unlikely in this case).

Hi Donald,

As Chris mentioned, your fix looks perfectly fine (unless there are some specific circumstances not mentioned in the scenario). It fits nicely into what the Judging at Regular REL document suggests:

Originally posted by JAR:

A player sees a card in a library that they shouldn’t have seen

Shuffle the card seen into the unknown portion of the library. If any player legally knows the position of any cards in the library, ensure that your fix preserves the positions of those cards.

Originally posted by JAR:

A player makes an in-game error not mentioned above

This will cover the bulk of player errors, and usually the least disruptive option is to leave the game as it is after fixing anything that is currently illegal (e.g. an Aura enchanting an illegal permanent). If the error involved a player forgetting to draw or discard cards, have them perform the appropriate action now. Otherwise, if the error was caught quickly and rewinding is relatively easy, you may undo all the actions back to the point that the illegal action happened. This can include returning random cards from the hand to the library to undo card draws (though don't shuffle the library if you do this), untapping permanents and undoing combat. This can be very disruptive where lots of decisions have been made or hidden information has been revealed since the illegal action, so don't go crazy with this!

I agree with all the answers here - just shuffling the cards into the random portion of the library and undoing the casting of the spell seems great.

The only thing I'd add here is that, if possible, it would probably be better for someone else to make the call in your match. A judge making a ruling in their own match is not the best of all possible things. There are plenty of cases where this would be fine, like if you already know your opponent well or if you're the only person in the room who has heard of the JAR, but if you're lucky enough to be playing FNM at a spot with two judges playing I would recommend asking the judge to come over rather than fix your own issues. That can help prevent any possible suspicion of misconduct.