Example:

touch in.h
gcc -x c -include in.h - -MD -MF /dev/stdout <<<'int main(){x; return 42;}

fails as it should.

Changing -MD to -MM causes the failure to go undetected (no stderr output, no nonzero exit status), making it look as if the compilation succeeded.

(Notes: Changing -MF /dev/stdout to -MF regular_file makes no difference.
Clang has this behavior too)