I cast Reanimate, it pulls up the Your Graveyard/Opponent's Graveyard options, with the default view to my graveyard. The only option in my graveyard is Grist, which I select, as it's a creature while it's in my graveyard. The spell resolves, and returns my opponent's Questing Beast to the battlefield.

This was especially bad because I had earlier in the turn just Unburial Rites to return my own Questing Beast to the battlefield, forcing me to sacrifice one to the legend rule, and lose 4 life instead of 3, and was no longer able to answer my opponent's Elder Gargaroth token by using Grist, as I should have had a round of priority to activate Grist to sacrifice my Questing Beast using the -2 ability. Instead, when my opponent received priority after I sac'd my second Questing Beast to the legend rule and proceeded to end the turn, he removed it with Parallax Wave, ending the game for me.

I can't say I wouldn't have still lost if it played out correctly, but I certainly had no chance when it played out as it did.