Never attribute to malice that which is adequately explained by stupidity.
I ran into this quote, called Hanlon's Razor, in an episode of the detective series New Tricks. It was new to me: as far as I can remember I'd never heard it before. However, it is sufficiently like one of my own favorite sayings that either (1) I heard it somewhere and filed it away subconsciously, or (2) it's so obvious that there must be dozens of variations on the same theme, or (3) I'm just brilliant.
Wikipedia attributes this form to Robert J. Hanlon, from about 1980, though, as I suspected, the idea's been around a long time. If I did pick up the idea subconsiously, I suspect it would have come from the Robert A. Heinlein story referenced in that article.
Here is one of my variations on the idea:
Most so-called conspiracies can be more readily explained by simple human stupidity.