So my opponent cast Mystic Confluence bouncing 3 creatures, but it didn't resolve due to "not having valid targets". Upon inspecting the game log, we deduced it was not a misclick by the opponent - the card just let him choose more than 3 creatures to bounce, so he chose some of them twice and then it fizzled, bouncing no creatures. (There was no interaction with Confluence on stack whatsoever, no reason for it to not work as expected.)