[Toadling]

It sounds like you have a stale client that's not syncing. Try going to OmniFocus' sync preference pane and clicking the "show clients" button. Are all the clients listed there valid and have they synced recently?

OmniFocus retains all the transactions until all your clients have synced. If you have a stale client that rarely or never syncs, your transactions will continue to pile up. The solutions is to either sync or simply unregister the stale client.

In the v1.8 sneaky peek, OmniFocus will automatically unregister stale clients (those that haven't synced in over 3 weeks). But this functionality isn't in the official release yet.

Hope this helps.

-Dennis