Quick Definition: Never attribute to malice that which can be adequately explained by stupidity
From Rational Wiki:
Hanlon’s razor is essentially a special case of Occam’s razor. Occam’s razor states that, assuming equal explanatory power, the simplest solution (formally, the one with fewest assumptions) should be preferred. Assuming intent is a pretty big assumption, but we all know that (other) people are idiots. The razor is most often invoked in the context of trying to refute aconspiracy theory: where a conspiracy is perceived, with no other evidence available, it is more likely to be ineptitude or apathy than malice that results in the problem.