Not valuable stamps, but some US postage I sold on
ebay, and then couldnt find the item to send to the buyer. Luckily he accepted a substitute lot, as it was only for postage anyway, and he wasnt a stamp guy.
The missing stamps turned up about 6 months later, deep under a pile of other material. My problem is that things get lost whenever I'm forced to do a periodic clear-up. If I never cleaned up I would never lose things!