It's easy to imagine having a compiler upgrade then add new warnings, that cause unchanged code in a project with -Werror to no longer build, thus breaking it.
Of course it can be argued that such "exposure" flushes out bad code with a vengeance, but it can also be considered to be annoying.
I think one was about superfluous parentheses, and the other about not parenthesising an expression because it assumed the programmer would be too stupid to know the precedence of operators.
I understand the noble goal of being warning-clean, but I wish there was a way to implement it in such a way that it could also be globally disabled. Then I could get a top level build, but the local resolution of the issues would still be on the owners of the code.
But all of this requires adding complexity and branching to the system. It can be done and is worth doing, you just have to choose your battles.
Not in OpenBSD it isn't.