Comment 10 for bug 792407

http://www.percona.com/doc/percona-xtrabackup/innobackupex/replication_ibk.html

"Using this option is always recommended when taking backups from a master server."

Maybe I forgot something now, but why? Shouldn't matter either way on a master?

http://www.percona.com/doc/percona-xtrabackup/innobackupex/innobackupex_option_reference.html#cmdoption-innobackupex--no-lock

I think here you should add a sentence that hints to using --safe-slave-backup:

"If you are considering to use --no-lock because your backups are failing to acquire the lock, this could be because of incoming replication events preventing the lock from succeeding. Please try using --safe-slave-backup to momentarily stop the replication slave thread, this may help the backup to succeed and you then don't need to resort to using --no-lock.