The number of people I have interviewed that have used C and C++ for years and can't tell me what undefined behavior is is too damned high. I've had one or two, and only one of them could give me a coherent answer of what the compiler is allowed to do if it runs across it.

In my experience it's only the language lawyers who actually know what to look out for, and there are damned few of them.

Which is stupid since undefined and unspecified behaviors are hugely important to how C and C++ are used in the field.