Florian, Voldaren Scion trigger (possibly when triggers for zero?) shuffled both players graveyards into library
See Match #242906651, Game #774928170.
In this game, I had a graveyard with roughly 12 cards in it, and my opponent had 2. I had a Florian, Voldaren Scion trigger for zero, and it shuffled both of our graveyards into our library.
Strangely, I had had the Florian out for several turns, triggering for zero multiple times, and it was the third or fourth trigger where this happened.
Rot Farm#40704 commented
Had the same thing happen to me, also shuffled my hand into the library a few times.
When I saw the battlefield during Florian Voldaren Scion's ability , I couldn't select.