[Brian]

A couple things can cause this. One possibility is that there's more than one copy of OmniFocus active and trying to write to your database. (Proceed with much caution - shutting the other copy down is the safest thing to do.)

The other, more benign thing that would cause this would be that OmniFocus didn't shut down as expected last time - laptop battery ran out, there was a kernel panic and the machine rebooted, whatever - and the file we use as a 'hey, I have this database' flag didn't get deleted. In that case, overriding the lock is unlikely to cause problems.