Bloody Mary starts out as a "game" usually played by kids and a mirror. Two things have been known to happen.
First, the brain likes to play tricks. You see what you think you want to see and kids get freaked out all over the place
Second...you can open a dang portal and let anything through just by calling out and issuing an invite. I've been saying this a lot lately, but here I go again....we do NOT know what these entities are out there! Where do they really come from? What the heck are they? We can only go by history, assumptions, experiences. So, when people "play" with these kind of things, be it "just for fun" or not, you can actually summon something.
Magic is the act of moving energy and manifesting. We now know this is a fact. It is real. Please, peeples....don't play with these things. All of the "magical" items out there, including mirrors for magic, are tools, not toys. You better know what you're doing with them before you start summoning and manifesting things you can't put back in the portal.