Many years ago I left a rag in the valley when I was installing a new intake on a small block. Finished it and stood back to admire my handy work, wanted to wipe my hands then realised where I'd left it. I've done other dumb things like leave spark plug sockets on spark plugs, sump plugs out when changing oil, leaving tools in the engine bay, left my helmet at the pit and only found out when we got called to stage etc etc