I think repetition is the key to stupidity, but not repetition alone. I think what makes a mistake stupid is willful repetition in the face of consequences. The example someone used of a child jamming his finger into a socket would fit into this.
Typos would not because a) they aren’t willful, and b) in this situation, there really aren’t any real consequences. This would be a careless, rather than stupid, mistake.
There are times, however, when the carelessness can be a stupid mistake. This would occur when the carelessness results in consequences, and is repeated.
An example of carelessness resulting in a stupid mistake could be as follows: When planning a trip, I forget to arrange to have anyone feed my fish while I am away. Upon returning home, I find all of the little fellows dead. I’ve made a careless mistake. Obviously, I should learn to be mindful of this when planning trips in the future. I go out and buy more fish, promising myself that I won’t kill them. Everything goes fine until a couple of months later, when I go on another trip. Sure enough, when I return home, these fish are dead, too. I forget to arrange for them to be fed–a stupid mistake, since I knew that this was something I should be careful of. Of course, anyone who would do this is too stupid to have fish anyway.
(Sorry this was so long, but I wanted to make sure my points were clear.)
“It is more from carelessness about the truth than from intentional lying that there is so much falsehood in the world.”