If I ask most programmer or people of other domains whether all months have more than 27 days I think most of them would say, of course all months have more than 27 days. That if wednesday is the second of a month the following friday out to be the 4th of the months.
You actually need to know about the strangeness of september 1752 to know that there this one month in the calender who had less days. It had no days between the 2nd and the 14th.
Children get taught in school that minutes have 60 seconds and not that there a body that decides 18 months in advance whether the minute is supposed to have 59, 60, 61 or 62 seconds.
This is no different from asking someone the fastest route to get to the store and being told "go a mile down that road and take a left" even though the person didn't check to see if the road was temporarily blocked. If you ask someone directions, they're probably not going to add "unless the road isn't temporarily blocked" and "unless a meteor hit the store last night and I didn't know about it yet" and "unless there's a quantum fluctuation that will move all your molecules right next to the store".
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.