In fact, I think you can tell a competent software developer from an incompetent one with a single interview question:
What's the worst code you've seen recently?
If their answer isn't immediately and without any hesitation these two words:
My own.
Then you should end the interview immediately. Sorry, pal.
It seems to me that few programmers would answer the question this way, competent or not. And it's probably not true for most competent programmers... I wonder if Jeff has actually received this answer from a interviewee.