“Never attribute to malice that which is adequately explained by stupidity.”

Murphy's Law Book Two : More Reasons Why Things Go Wrong! (1980) ISBN 0843106743 by Arthur Bloch page 52.
This statement has often been referred to as Hanlon's Razor, though the author himself has remained relatively unknown, and some theories arose that "Hanlon" might have been a corruption of "Heinlein". Hanlon actually was a winner in a contest to come up with further statements similar to "Murphy's Law", for publication in this particular book. This phrase or very similar statements have also been attributed to William James, Napoleon Bonaparte, Richard Feynman (who might well have quoted it) and others.
Similar statements have been made by Goethe, and indeed, Robert Heinlein:
In The Sorrows of Young Werther Goethe declared, "Misunderstandings and neglect occasion more mischief in the world than even malice and wickedness. At all events, the two latter are of less frequent occurrence."
In his story Logic of Empire (1941) Heinlein declares: "You have attributed conditions to villainy that simply result from stupidity". He calls this the "devil theory" of sociology. His character Lazarus Long also voices a variation on the theme in the novel Time Enough for Love: "Never underestimate the power of human stupidity."
Variants of the phrase which have been variously attributed to this wide assortment of authors include:
Never ascribe to malice that which can be adequately explained by stupidity.
Never ascribe to malice that which can be explained by incompetence.
Never ascribe to malice that which is adequately explained by stupidity.