Hanlon's razor

Hanlon's razor is an adage, most commonly attributed to one Robert J. Hanlon, which is generally stated as: Never attribute to malice that which can be adequately explained by stupidity.

Sometimes "incompetence" is used instead of "stupidity".

Guises and corollaries
Therefore you speak unskillfully, or, if your knowledge be more, it is much darkened in your malice.

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 big assumption, but we all know that (other) people can be, and seemingly more often than not are, idiots and even intelligent people make mistakes. The razor is most often invoked in the context of trying to refute a conspiracy 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.

The statement has also been seen in the guise of "cock-up before conspiracy", based on a statement about the media and government by Margaret Thatcher's press secretary, Bernard Ingham. The Wikipedia rule is a nicer rephrasing of this rule, noting that idiots at least have their hearts in the right place, despite being ten times as damaging.

The razor also has a corollary in Clark's law (named after J. Porter Clark, and based on Arthur C. Clarke's Laws), which states that "any sufficiently advanced incompetence is indistinguishable from malice". A further observation was made by commenting on the fact that idiocy is all pervasive, whereas intentional malice tends to go away occasionally. Dumas appeared to be quite sure of which he preferred to happen:

I prefer rogues to imbeciles, because rogues sometimes rest.

It appears to be something Dumas said, not something he wrote, but is a fairly true point (and a very popular quote). German general Kurt von Hammerstein-Equord came up with a still more elaborate version:

More completely
Cribbing from a LessWrong commenter:
 * Never assume malice when stupidity will suffice.
 * Never assume stupidity when ignorance will suffice.
 * Never assume ignorance when forgivable error will suffice.
 * Never assume error when information you hadn't adequately accounted for will suffice.
 * Therefore, never assume malice when information you hadn't adequately accounted for will suffice.