[Brian]

OmniFocus and iCal both send data to a piece of the OS called Sync Services; sometimes things don't work because we did the wrong thing, sometimes iCal does the wrong thing, and sometimes Sync Services just does the wrong thing all by itself.

Unfortunately, we can only fix the problems that occur on our end - the other parts we don't have any influence over.

You should send us an email from the Send Feedback... item in the help menu; we'll gather some information and try to determine what's going on here.