This scenario spawned from a PPTQ I ran yesterday at Cosmic Comics in Bellingham, Wa. Fortunately, in the real example Sammy's warning came in time, but the entire store was discussing what would happen if it didn't.

You are head judging a Standard PPTQ. Alan is playing Green/White devotion and is in the deep into game two with limited time remaining on the clock. He has just resolved a Genesis Hydra where X=21. He places his card of choice, Mastery of the Unseen, onto the battlefield. Then Alan accidentally picks up his sideboard, which happens to be about the same size as his library, and takes the remaining 20 cards from Genesis Hydra. Sammy the spectator shouts “Be careful!” but alas the warning comes to late and as Alan has just mashed the cards from Genesis Hydra into his sideboard.

Has Sammy done anything wrong?
What penalty should Alan receive?
How do you fix this mess?

Has Sammy done anything wrong?

Nah, not really. He didn't give any game information or anything like that.

What penalty/how to fix?

Is Alan's deck legal with his sideboard shuffled in? Is it what he
presented?

No, it isn't, so that's a Deck/Decklist Problem and will be a Game Loss. If
this is Game 1 you can have a talk about figuring out what the sideboard
was from the decklist, but I wouldn't. The infraction is clear to me and I
see no reason to deviate.

I'm actually feeling like this is more of a GRV - cards that were supposed to go one place (the library) instead went somewhere else (the sideboard). There's a pretty simple backup associated as well, considering that Genesis Hydra reveals all cards, although 21 is a lot of cards to remember so that might complicate things.

From the IPG section, Deck/Decklist Problem:
“If the sideboard is not kept sufficiently separate from the deck during
play, it becomes impossible to determine the legality of the deck.”

Once cards from the sideboard are mixed with cards from the rest of the
cards, it is D/DL and a GL.

Fair enough. I should read more carefully next time.

It happens more than you would think, or at least similar. In GP Liverpool someone accidentally shuffled his deck into his opponents after resolving a fetchland (all had channel fireball sleeves).

Ha! How did you resolve that one?