Forgive me if I'm being overly simplistic here but did you try the "Undo" button in the toolbar? That should have worked for this type of event, if not please email the ninjas at
omnifocus-ipad@omnigroup.com so we can help you out with that. Also, you can tap the view menu (the eyeball icon) and choose "All" to see your completed actions and recover the accidentally completed action that way. Hope this helps!