Most languages supported by CC Mode have two styles of comments, namely block comments and line comments. Your project may have such a strong preference for one of them, that you wish “wrong” style comments to be clearly marked.
You can get CC Mode to do this by setting the default comment style,
if necessary, (see Minor Modes) and setting the customizable
option c-mark-wrong-style-of-comment to non-nil.
When this customizable option is non-nil, comment delimiters
which aren’t of the default style will be fontified with
font-lock-warning-face.