At FNM my roommate told me of how his opponent accidentally shuffled his hand into his library (how, I don't know). How do you fix this?

At Competitive REL, we've consistently ruled that this is a GRV, but there's no remedy to restore the hand; the player gets to continue with no cards in hand.

At Regular REL, if there is evidence to support it, you could consider trying to reconstruct the hand, or at least the part everyone can agree on. In a lot of cases, that just won't be possible, as only that player can say what was in his hand. Allowing them to name a selection of cards to put in hand just seems like a bad idea.

Thanks! – Scott Marshall, Judge Program NetRep, L5, Denver
New to judging? <http://wiki.internationalmagicjudges.net/index.php?title=Welcome_to_Level_1>
Looking for something (judge-related)? <http://www.internationalmagicjudges.net/>

I actually had this happen to me at a Prerelease (about 14 years ago). My opponent drew for turn (turn 3+), looked at his hand, then put his cards on top of his deck and began shuffling. Surprised, I asked what he was doing. “Mulliganing” he said. I almost dislocated my shoulder raising my arm so fast and yelling “JUDGE!”. I believe the judge ruled to continue play with no hand which led to a quick concession.