Never attribute to malice anything more easily explained by stupidity.
"Never attribute to malice that which is adequately explained by stupidity."
Robert J. Hanlon, Murphy's Law book two: More reasons why things go wrong! (1980) ISBN 0843106743 by Arthur Bloch page 52.
Wikipedia:
According to Joseph Bigler, the quotation first came from Robert J. Hanlon of Scranton, Pennsylvania, as a submission for a book compilation of various jokes related to Murphy's law published in 1980 entitled Murphy's Law Book Two, More Reasons Why Things Go Wrong.[1] The name was inspired by Occam's razor.[2]
A similar quotation appears in Robert A. Heinlein's 1941 short story "Logic of Empire" ("You have attributed conditions to villainy that simply result from stupidity"); this was noticed in 1996 (five years before Bigler identified the Robert J. Hanlon citation) and first referenced in version 4.0.0 of the Jargon File,[3] with speculation that Hanlon's Razor might be a corruption of "Heinlein's Razor". "Heinlein's Razor" has since been defined as variations on Never attribute to malice that which can be adequately explained by stupidity, but don't rule out malice.[4] Yet another similar epigram ("Never ascribe to malice that which is adequately explained by incompetence") has been widely attributed to Napoleon Bonaparte.[5] Another similar quote appears in Goethe's The Sorrows of Young Werther (1774): "...misunderstandings and neglect create more confusion in this world than trickery and malice. At any rate, the last two are certainly much less frequent."