A month ago I stupidly managed to lose my wallet. I normally keep it in a fixed place (like most of my stuff) so that I can keep track of it - specifically, it lives in the front pocket of my rucksack. If I shift it to a pocket, I put it back there afterwards, otherwise it will wind up at the bottom of a pile if I just "put it to one side for a minute". (This is how I locked myself out a while back, when I did something similar with my keys.) Anyway, I know I had it when I went shopping, and I put it in my back pocket, so I assumed that it fell out when I was on the bus, i.e. when the pocket became horizontal rather than vertical; this is why bum bags are practical, even if they're not fashionable! Anyway, I looked around for it, and I checked my bag several times. In fact, I emptied it out and unzipped everything, just in case I missed it when I was peering in from above. No joy, so I cancelled my credit cards etc. The bank told me that I needed to go along to the police station to report this, so that I could get a crime number; I did tell them that this was my own fault, i.e. it was lost rather than stolen, but apparently this was necessary in case anyone used the cards fraudulently. However, I never actually got round to doing this, partly because I'd feel a bit guilty about wasting police time with this when they've got more important things to deal with.
Last Friday I was at work, and wanted to get something out of my rucksack, so I just stuck my hand in and groped randomly rather than actually looking at what I was doing. I was quite surprised to pull my wallet out! The obvious explanation is that it had in fact been in my bag the entire time, but I'd somehow failed to notice it, even when I was explicitly looking for it. The alternate explanation is that it wasn't there until I reached in without looking at it, or for it; not quite "Schroedinger's rucksack", but something along those lines. I recall something similar from the Narnia stories - the doorways (e.g. the wardrobe) would only work when you weren't looking for them. I'm not actually accusing Mr Tumnus of pickpocketing me via an interspatial portal, but it's weird. Longtime readers may recall something similar that happened to me in my youth, when I duplicated a Skudo token.
Ah well, I got my money back, and it's prompted me to update my Sentinel records, so there's a happy ending :)