A "minor slip" is open to interpretation. That doesn't mean something minor could have massive consequences. I'd consider forgetting to activate shield oath a minor slip up. It's easy enough to do but can have massive reprecussions.
I'd bet most paladins have forgotten to activate shield oath at one point or another. We're all human and prone to err from time to time no matter what we do (in the virtual world or in the real world). Warriors at least notice their heath pool is low without defiance activated.
