Theists, the world over, claim that God is omniscient. However, this is not an easy claim to make for a whole host of reasons, one of which is worth looking into here. I want to look at the idea that in many instances, you cannot know that you don’t know something. If there is a situation where you cannot know something, then if it is claimed that you are omniscient, this would invalidate that claim.
For example, there could conceivably be something that God does not know. Conceivably, perhaps another dimension run by another God exists that does not coincide at all with this dimension. If one eternal God can exist, why not another in an entirely different dimension and unbeknownst to the first God? Now, it is unimportant as to whether this is possible or not. What is important is that God could not know that he did not know this by the very nature of not knowing it!
Where does this leave God? Well, God is in a situation whereby he cannot know that he knows everything. He might think he knows everything. Epistemologically speaking, though, he cannot know it. There's always a chance he's an experiment in an elaborate lab, programmed to think he's omnipotent and omniscient (yes, God could be plugged into the Matrix and he’d never know it!). There's a chance he's one of a trillion gods in a trillion different universes etc. etc.
It only takes one thing you cannot know to invalidate omniscience. God cannot know that he knows everything.