You may be pointing out a problem with English. "Programmers" could simply mean more than one programmer, but there's an implication that all programmers have those problems or perhaps that the problems are pervasive.
I get the impression that these are mistakes that someone has seen (or made) at least once. Some of them may mostly be made by programmers who are beginners.
I agree that it would be nice to get percentages, even if there are large error bars. I think the main point of these lists is to warn programmers that they may need much more specific knowledge than they think they need. For what it's worth, the commenters on those lists are programmers, and I'm not seeing comments which say "That never happens!".
There are some long lists of false beliefs that programmers hold. isn't because programmers are especially likely to be more wrong than anyone else, it's just that programming offers a better opportunity than most people get to find out how incomplete their model of the world is.
I'm posting about this here, not just because this information has a decent chance of being both entertaining and useful, but because LWers try to figure things out from relatively simple principles-- who knows what simplifying assumptions might be tripping us up?
The classic (and I think the first) was about names. There have been a few more lists created since then.
Time. And time zones. Crowd-sourced time errors.
Addresses. Possibly more about addresses. I haven't compared the lists.
Gender. This is so short I assume it's seriously incomplete.
Networks. Weirdly, there is no list of falsehoods programmers believe about html (or at least a fast search didn't turn anything up). Don't trust the words in the url.
Distributed computing Build systems.
Poem about character conversion.
I got started on the subject because of this about testing your code, which was posted by Andrew Ducker.