Well, karma doesn't exist as a fundamental law of the universe. If you steal a candy bar from a store and nobody ever learns about it, nothing bad is going to happen to you because of it.

Karma exists only as a social phenomenon. If you act like an asshole, people are gonna act like assholes right back to you, and if you act nice, people will be friendly to you. So be nice if you want others to be nice to you. If you've already pissed somebody off, try apologizing and somehow making up for having wronged them.