A simple case is the warning clang will generate for:

  int err = 0;
  if (err = f()) {
  }

This is indeed a useful warning, as it's common to mistype == as = in this situation, but it's also absolutely correct code, and the idiom that silences the warning (surrounding with extra parentheses) does not change anything about the correctness of the code.

Unintended code (like having both arms of an if-else-statement execute the same action, or a duplicate condition in a logical expression) is sometimes perfectly valid as far as the standard goes. A double condition for example may simply be a result of poor refactoring, and thus the program even works as intended – however, the compiler cannot know that. That's why the helpful warnings flag those statements – -Werror then makes the compiler abort the compilation altogether on any warnings which is clearly not standard-compliant in cases like the one I outlined.

The majority of the warning options showed in the article are intended to find unintended code (which can however perfectly defined if not intended behaviour as opposed to invalid, undefined behaviour). The very basic examples showed for those options also themselves do not contain anything that by itself would make the code not standard-compliant.