IF the description of the falsehood includes the line "Obviously, this isn't necessarily true." I am going to go out on a limb and suggest that programmers don't really believe it - at any rate I would like documentation of programmers believing that people have only one phone number.
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"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.
