I think we probably all have those kinds of days every now and again.
I was putting a 390 together once and I always fill the engine and prime on the engine stand, without the intake on, etc.
I can't remember exactly which oil I was using, but it had the little foil caps that you have to tear/cut away when you unscrew the cap to the bottle. I remember that I took my knife and cut the foil tab up so that there was a big hole, but the tab was still attached to the bottle.
I was pouring away when I saw the foil piece detach from the bottle and swim right towards a drain hole in the lifter valley. It disappeared.
Couldn't see it at all, so I had to drop the pan.