So it has happened to me a bunch of times, but most recently..... Dr Wu's purple dark energon sword and 2 titan masters. I don't really mind the two titan masters since they are only 5 bucks at target, but I know once I get them, within 36 hours i'll find them. If I do, i'll probably take some black spray paint to them and make an evil army or something like that. another recent thing I did was, I got the Animated Optimus Ax on shapeways and for like 3 months I couldn't find it, so I assumed it was gone forever. So I ordered a new one, of course once I ordered the new one, I found the old one. It wasn't so bad because I just armed Hosehead with one and Hot Spot with the other. I'm toying around with the idea of getting 2 more, one for Fire Convoy and one for CHUG Inferno.
If I know I have something, I don't rebuy it. If I can't find it but know I own it, I look for it. That usually does the trick.
This happened to me with minicons. Sea and Race minicon teams I had as a kid and kept them in decent condition. Moved and couldn't find them with the figures I had brought with me and ended up buying sets off ebay / here on the forums of both. Found them a couple months ago, Sea team sans missiles and Race time minus one of the members, so wasn't too much of a loss and I was smart enough to get the recolors so at least there is that as well.