'file not found' only effects the uploads also. it means that someone sent a request for a specific file index (every shared file is given a certain index) but gave the wrong filename for that index. there's two reasons this can happen: one is because the person who is serving the file happened to hit 'refresh' in the library, causing limewire to reorganize its library. the other is because there's a problem with the person sending the request -- it got confused somehow.

like i mentioned earlier with the XYZ.doc and ZYX.doc answer, limewire doesn't use index or filenames anymore when it doesn't have to -- it uses the unique hash of the file (when it can), which means that even if someone hits refresh and the library is reorganized, the request will still work. it also lowers the chance of buggy requests, because all you have to keep track of is the hash, not the name and index.