I've posted this before but it's a classic.

I had cycled to my usual birdwatching spot and when it was time to leave realized I had just one cycling glove on. I looked everywhere -- seemed odd because I usually don't take gloves off.

The glove was nowhere to be found -- until I got home and discovered it on the floor. I had apparently left home with just one glove.

I will say in defense of my sanity, it was summer. I hope I would have noticed sooner in winter.