My big weakness is the "mystery boxes" from ebay. You know - the blurry picture, description something like "lots of old camera stuff - don't know if it works or even what it is?"
Last time I couldn't find my cable release I bought one of those for a winning bid much less than the postage. I now have 12 cable releases in a wide range of lengths! And a single-plate adapter for international back - one each in 9x12cm, 4x5", 10x15cm and 13x18cm. A big stack of various lens boards. About 30 different lens hoods. And lots of the "don't know what it is"-es.
Iv've just bought another one of those packs - I know there's two iris lens mounts in it, and "some other stuff like lenses".