There's per-asset locking to maintain integrity in the system.  Unfortunately there are still some situations where something fails, and the lock does not get removed.  With the site in beta, I have to carefully inspect the logs when these failures happen, then address the issues in code.  In this case, I believe it was an uncaught exception while talking to bitcoind on a trade.  I've plugged in some code to address it.  Hopefully that helps prevent it in the future. 

Cheers.