This blog post from a few years ago suggests that using BitTorrent services over Tor is a bad idea for multiple reasons. However, it also suggests some fixes to get over these problems.
Is using a
It doesn't make you anonymous. Torrent protocol wasn't designed with anonymity in mind and there are a million ways you're going to leak your actual IP address.
Tor is a TCP only network.
While this doesn't give you the anonymity you wanted, it will hurt the network for other users.
Edit: I misread the statement. Thanks for pointing this out to all the repliers! My fault.
The first point of the answer is misleading.
Tor is indeed designed with anonymity in mind.
The leaks occur in different layers, like the for e.g. chosen BitTorrent client.