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.” 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, Ninety-Ninety Rule.
At http://www.statusq.org/2001/11/26.html it is claimed that Hanlon's Razor was coined by one Robert J. Hanlon of Scranton, PA. However, a curiously similar remark (“You have attributed conditions to villainy that simply result from stupidity.”) appears in Logic of Empire, a classic 1941 SF story by Robert A. Heinlein, who calls the error it indicates the ‘devil theory’ of sociology. Similar epigrams have been attributed to William James and (on dubious evidence) Napoleon Bonaparte.