Hanlon's Razor /prov./ A corollary of Finagle's Law, similar to Occam's Razor, that reads "Never attribute to malice that which can be adequately explained by stupidity."
The derivation of the Hanlon eponym is not definitely known, but a very similar remark ("You have attributed conditions to villainy that simply result from stupidity.") appears in "Logic of Empire", a 1941 story by Robert A. Heinlein, who calls it the `devil theory' of sociology.
Heinlein's popularity in the hacker culture makes plausible the supposition that `Hanlon' is derived from `Heinlein' by phonetic corruption.
A similar epigram has been attributed to William James, but Heinlein more probably got the idea from Alfred Korzybski and other practitioners of General Semantics. Quoted here because it seems to be a particular favorite of hackers, often showing up in sig blocks, fortune cookie files and the login banners of BBS systems and commercial networks. This probably reflects the hacker's daily experience of environments created by well-intentioned but short-sighted people. Compare Sturgeon's Law.
It is a very good day when I see these two cited. :)