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The longest time was for a couple books I'd lent to a friend some years ago (six, to be exact) and forgot lending out. Couple weeks ago I was visiting him in N.Y. and he handed me a bag of books which I thought I had "misplaced." It's not like it was the first time I'd seen him in all that time either, I just never remembered to ask him about the books and he never said anything.
The most ridiculous incident was losing my coat at school a couple winters ago. It was really, really cold out, but somehow I completely failed to notice I wasn't wearing the coat. Two weeks later a professor of mine asked if i'd lost a coat and told me he'd found it after one of his classes. Totally at random.
I have a bad habit of emptying my pockets (of keys, loose change, wallet, papers etc.) whenever I enter a room, and somehow hiding them without even being aware of it. Usually there are only a set number of places that I unconsciously gravitate to (the top of the bookshelf, behind the books, in the flower pot etc) but I've also come up with some really stupifying and illogical hiding spots. I once hid my wallet in a stack of old newspapers and forgot about it. It kept me from going out with friends that night because I didn't want to drive anywhere without my license, and took all weekend for me to find it. Pretty retoidid of me.