Note to self 009 - it's called Hanlon's razor

And it goes: Never attribute to malice that which is adequately explained by stupidity.

Although sometimes Heinlein's Razor or Napoleon's unnamed version of it seem more appropriate...

Heinlein's razor:
Never ascribe to malice that which is adequately explained by incompetence, but don't rule out malice

Napoleon's unnamed epigram:
Never ascribe to malice that which is adequately explained by incompetence.

See http://en.wikipedia.org/wiki/Hanlon's_razor