By trap I mean a surprising exception to the common rules. Most high school graduates learn algebra, yet they rarely learn how to avoid accidental division by z

By trap I mean a surprising exception to the common rules. Most high school graduates learn algebra, yet they rarely learn how to avoid accidental division by zero. It's a surprising exception. "null != null" is also a surprising exception to the common rules. I honestly wouldn't expect most developers to know it until their code breaks because of it. That doesn't mean the surprising rule is wrong, it just means people have to learn it. Once they learn it, they may try to find the deeper model where the rule is no longer a surprise but rather an important feature.