Why is this artist mad?

[I Love Mr Slope]

This was painted by a person with a rare and severe mental disorder. He was constantly seeing his own fantasies all around him. He also had a certain phobia (undisclosed).
His (the poster’s) psychiatry professor showed this painting in a lecture, and said there was one tell-tale sign in it that showed the painter’s insanity.
The professor didn’t say what that sign was, leaving the students to do the guesswork. The only clues he gave was, “don’t look for small details, look at the whole; if you figure out what the phobia was, you’ve got the answer; ask yourself what could have preceded this scene; think of what the place would look like with all the objects removed“.
The professor said that during the 15 years of his teaching, only one student had figured it out.