Okay, this glitch amazes me a lot. It may be known already, but I want to point it out just to see if anyone had this problem.
Summon something --anything. (a Monster)
Then from the menu, go to the title and then load another save file. Your summoned monster will still be there O_O. Which means if players caught onto that and were clever, at the same time wanting a second play through, they'd probably do something like this.