"falsehoods programmers believe about X" is shorthand for "assumptions about X that some programmers have not examined prior to deployment in production code".

"falsehoods programmers believe about X" is shorthand for "assumptions about X that some programmers have not examined prior to deployment in production code". Any system that has an input with a single "telephone number" field that is both required and cannot accept multiple entries fits this pattern. Think of it as a list of antipatterns to be avoided. Some of them were deployed intentionally, and others pop up because a programmer took the spec they were handed and implemented it without thinking about it -- or they did think about it and their attempt to fix it was overruled.