Half a door.
Our last house had a few boards across the rafters in the attic, most of which weren't much use as flooring because they didn't match the spaces very well so you had to be careful not to stand too near the edges. Then there was the half-door that kinda-sorta acted as a bit of flooring, except it was even more useless because you couldn't stand in the middle either because that's where the weak panel was. And at first I couldn't even get rid of the nasty flooring because I couldn't figure out how to get it through the hatch into the attic. I'm not sure if it was something left from when the house was built, or if someone had work done on the roof and took the chance to get some panels in there. Before long I was the proud owner of a jigsaw, which made the problem much easier to handle.
Most of what I've found in our current house has been evidence of historic rodent activity. Long-abandoned mouse nests in the walls and the like. I found what I think is a complete mouse skeleton in the basement. It looks like it's in pretty good condition too. I'm not sure what I'll do with it. For now it guards a brick in a dark corner.