WIST

Never attribute to malice that which is adequately explained by stupidity.

Robert J. Hanlon (contemp.)
“Hanlon’s Razor,” Murphy’s Law, Book Two (ed. A. Bloch) (1980)

Cf. Heinlein and Napoleon. See here for more information, including discussion that "Robert J. Hanlon" may be a corruption or obfuscation of "Robert A. Heinlein."

Various variants in combinations with "ascribe," "what can," "incompetence."