As I was preparing for a Saturday morning commute, I noticed that my giant Kryptonite NY Long Shackle lock was not on my rear rack. There's no reason I would have removed it when I got home and it was nowhere in the garage.
A-ha! After unlocking my bike the night before, I placed it on an object near the bike rack and began attaching all my lights & generally getting set up. I rode off without it.
Luckily, the Forces of Stupid Human Thievery subsided for a few hours and it was right where I left it when I rode back. I don't know what someone would do with an unusable U-lock, but you never know.