Since we're doing quotes:
Never attribute to malice that which can be adequately explained by stupidity.
-- Hanlon's Razor
Of course, once you start thinking in terms of stories instead of theories, it's easy to forget that only a tiny fraction of things that happen are the result of planning or motive.
(Since there didn't seem to be one for this month, and I just ran across a nice quote.)
A monthly thread for posting any interesting rationality-related quotes you've seen recently on the Internet, or had stored in your quotesfile for ages.