See also Regehr's example[1] where a formally verified C compiler generates incorrect output because of an inconsistent value in <limits.h> (TL;DR: The compiler can pick whether "char" is signed or unsigned. Compcert picked one, but the linux system header used the other for CHAR_MIN and CHAR_MAX).

1: https://blog.regehr.org/archives/482 there were many issues here, not just with compcert