There's actually two unrelated problems that could cause 0KB/s uploads in the current LimeWire version (2.5.5).

One is if you're throttling bandwith and the uploader has their HTTP keep-alive enabled (generally for clients that want to upload a portion of a file, then upload another portion, etc..). In this case, the upload gets stuck in a sort of infinite sleeping-loop, and the upload is considered part of the 'active uploads' -- so if you have a max limit of 10, only 9 other uploads will be allowed (until you kill the upload).

The other is uploaders that somehow manage to instantly disconnect, and the core cleans up all the related statistics (meaning it doesn't use an active upload slot), but the GUI still displays it as uploading, because somehow, the uploader's status never changed.

I've found & fixed the first problem (although a temporary workaround would be to disable upload throttling [ie: set 100% allowed] ). I have a workaround for the second problem, but I'm absolutely stymied as to how it could even happen.