Well... Mu-san told me that once, monsters showed up and started to attack people, but they were taken care of... And that they found themselves mixed up in other peoples' realities instead of their own. And recently, a lot of everyone's friends have gone missing from their realities and showing up somewhere else... but from what I've heard, without their regular appearances or memories.

More has probably happened than just that, but those are the only things that I've heard of.