Also, Pandemonium won't target any creature that is the result of an earlier PU. You can even PU a creature with an actual PU, and then cast Pandemonium, and Pandemonium will skip that creature.
Wierd, but it's how Zanz avoided the rare but possible several-PUs-of-the-same-creature-over-and-over problem that Pandemonium's very first version had.