Okay, normally I prefer to have a favgroup ready for this sort of thing, but this kind of thing is really hard to find a good all-purpose search for populating, so we're going with this strategy until I can pull a big enough one for it.
But basically, the tag concept is as defined in the title: an otherwise normal image that has something very much not normal happening in the background. For example: post #9795946. Looks like a pretty normal group shot of various Limbus Company characters, until you look at what's going on behind the foreground characters, which depicts some rather amusing things. This is the sort of concept the tag would be covering. It's one of those common tropes I think everyone has heard of, right?
I'm honestly surprised we never got a tag for it to my knowledge, considering its ubiquity. But I'm going to propose it now to troubleshoot and stuff. I think the fact that I'm struggling to pull together a favgroup at all is evidence itself for the tag's existence, as if you want to see images depicting background events, you're pretty much out of luck right now. But what do I know? I want to go with "funny background event" as the name as that's how this trope is usually referenced in popular culture (not to mention most background events in art are intended to be amusing...), but I can be talked into a different name if this would cause issues.
What do we think?