Some versions prefer "incompetence" to stupidity, thus connotating short-sightedness rather than simple ignorance or wrongheadedness. It is not known who "Hanlon" is; possibly the name is a linguistic corruption of "Heinlein."
When anything is possible, mistakes are inevitable. If nothing in Puzzlebox happens by accident, then whatever is happening to you is because someone made it happen. The emotional response is to immediately assume maliciousness; the intellectual response is to immediately assume incompetence.