Something that's overlooked is either not seen or deliberately ignored. If you find a plastic egg full of stale jelly beans in June, it may be an overlooked prize from April's Easter egg hunt.
When you disregard, pass over, or fail to notice something, you can call it overlooked. Your best friend's overlooked faults might include her tendency to be late and her annoying laugh — you ignore both because you like her so much. This adjective stems from the verb overlook, which is currently defined as "fail to notice" but originally meant "peer over the top of" or "inspect."