Even if a bad programmer did 200 times as many interviews as a good programmer, that would mean that about half the programmers can't do FizzBuzz, which is still unsettling.
If your idea of a "bad programmer" is someone who studied programming, but had unimpressive results, then yes, the idea that half the programmers can't do FizzBuzz is unsettling.
However, the set of "bad programmers" also includes crazy people who believe they understand programming without any good reasons; overconfident people who used Excel for a few months and now believe they know everything there is about using computers; etc. It is not so difficult to believe that these people are as numerous as the real programmers.
In other words...
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.