I am running a time machine backup that suddenly kept giving "backup failed" error messages. The description was completely unhelpful and after having restarted and repair the disk solved nothing, I had to open the terminal n order to know what was going on.
sudo grep backupd /var/log/system.log
Ah finally an error code, a quick search on google gave this helpful page: http://www.bytebot.net/blog/archives/2008/08/13/fixing-time-machine-backup-failed-with-error-11
Turns out deleting /Time Machine Backups/Backups.backupdb/nameOfComputer/date.inProgress was enough :S. Thanks apl.