A man is wearing a neon yellow vest while leaning over a roof and another man is standing behind him.
Two men can not see each other because there is a wall in their way.
If the man is behind him the he should be able to see him
If one man is standing behind another, he can see the other man
Either one man is standing behind the other or there is a wall in their way, so they can't see each other. Both things cannot be true simultaneously.