They're forgetting things or burning food. Often the medical system unknowingly supports denial. For some people, things don't get better. They wonder why they're doing some silly things. They have these odd events and they keep rationalizing them away.
Denial can be very difficult for both family members and medical staff.
This can stir up huge arguments. The person in denial says there's nothing wrong, even when directly confronted by family members. Generally, very direct feedback is necessary. I often find that distraction, such as changing the topic of conversation, is better than getting into long-winded or violent arguments. For example, the head-injured person may want to drive when it's not safe to do so.