>Even young children instinctively figure out the 'it was merely an accident' excuse for their misbehavior. Following Hanlon's razor would have you outwitted by toddlers.
Hanlon's Razor is absolutely valid.
That said, I'd posit that stupidity isn't accidental and shouldn't be excused. Rather it should be stomped on just as hard as malice since, as others have pointed out, the effect is the same.