[Brian]

The piece of OS X which we use to sync with iCal is attempting to do a complex job, and sometimes... unintended things... happen to your data.

To be clear, I'm not talking about a bug in OmniFocus here. We can fix those on our end. There are whole portions of the process that are outside of our control - Sync Services, iCal, some other application which is also syncing...

In any case, OmniFocus' iCal sync is manual so the user can be on-guard for those sorts of changes to their data. They know when the sync happened and can check to make sure that everything went as expected.