People who are schitzophrenic often see or hear things that aren't there.
I don't have this. But sometimes I have this thought that I will be unable to hear and see things that are there. Like, what if I walk into my apartment and my roommate is there, but I can't see him? He's shouting at me, waving his arms, and I just walk right past.
This has never happened, but it's a thought I've had many times. I don't know what it means.