The user is cheating.

The server is overloaded and unresponsive. Just try to keep the session open until the server responds again. (Flooding the server with consecutive manual updates is not recommended.)

You are using a faulty client. If you want to use an experimental or CVS version, you can do it at your own risk.

If a torrent you are currently leeching/seeding is not listed on your profile, just wait or force a manual update.

Make sure you exit your client properly, so that the tracker receives "event=completed".

If the tracker is down, do not stop seeding. As long as the tracker is back up before you exit the client the stats should update properly.

Yes. The tracker now updates the stats correctly for all bittorrent clients. However, we still recommend that you avoid the following clients:

BitTorrent++,
Nova Torrent,
TorrentStorm.

These clients do not report correctly to the tracker when cancelling/finishing a torrent session. If you use them then a few MB may not be counted towards the stats near the end, and torrents may still be listed in your profile for some time after you have closed the client.

Also, clients in alpha or beta version should be avoided.