You're probably thinking of implementation-defined behaviour, which is the term for behaviour that, although not fully specified by the standard, is a valid behaviour for the program.

Undefined behaviour is different. From the article: "Undefined behavior gives the implementor license not to catch certain program errors that are difficult to diagnose."

Many people don't notice the distinction, and their intuition about what compilers should do with their code matches implementation-defined behaviour; and therefore they complain when, for UB, it does not behave this way.

There is a very strong argument for reclassifying many specific UBs as implementation-defined behaviours, but that's a rather different conversation from "the compiler should always do something sensible when encountering UB".