I've gone crazy looking for a hat that was already on my head or keys that were already in my hand, too.
Most common: I'd go to get something (can't think of anything specific, so I'll use soap as an example), got to where the soap was, and totally forgotten what I came to get. I'd wind up grabbing an apple or something (again, this is just an example). Then, when I get back to the bathroom, I'll think, "Why the hell did I just go get this apple?"
It's like I'm Leonard from "Memento" or something. It's very possible I've killed someone and forgotten about it!