That happens if you are refreshing your library. Every file has a certain position in your library, to identify it, in case you are sharing files with the same name. Some client received a response from you containing the file at its old position and is now trying to download it.

The problem is that some servents (Gnucleus, Bearshare) will try again and again, although they receive a 404 everytime they download. Eventually they will stop, but it can take them a couple of minutes.