Know-It-All Mac Computers
Did you know that your Apple Mac computer remembers everything you download, as part of a feature that quarantines dodgy files you try to download from the Internet? You can't readily "turn it off", and to purge the data you need to use the Terminal. From what I gather it is also backed up by default in Time Machine, but you can exclude it. The original story on OSX Daily is here, and an article on Macgasm goes into some more detail (and is a little less sanguine about this behavior). Some good comments on the Macgasm article too.