well basically it's shorthand for "things that do not apply to every locale for X of instance of Y, that programmers have decided to implement as if they do apply to every locale or instance either through not realizing they do not apply or that the non-applicability in some situations are such bizarre outliers in relation to the rest of the world that it has been decided to be an acceptable bug that they will not fix" but that's not very pithy.

But of course what happens is the outliers have to deal with these systems that treat them as bad data possessing impossible properties and then the curse and say hey these stupid programmers don't realize that the world is not just like X1 or Y1, but that us rare instances of X2 and Y2 also exist!

And then an expert with sufficient knowledge about all the outliers compiles a list entitled Falsehoods Programmers believe about {X}.