I've lost count:
Put the key to a gun cabinet in a "great" spot, ended up drilling the lock and finding the key 2-3 years later (it was a great spot).
Stood a barreled action up in the corner of an I beam pillar in the shop, looked at the same corner for 5 years swearing I left it there, then I looked on the other side of the I beam and there it was.
Got a phone call from my LGS saying my new SxS was in, I had to ask "What SxS? I had to check my e-mail before I believed him. I still don't remember bidding on it, let alone paying $500 for it.
And so it goes....