It doesn't matter how unified your parents are when it comes to rules, most kids have a few stories that fall under the "don't tell mom (or dad)" umbrella.
These rare instances of secret alliance with one parent make an experience that much more special. After all, everything is cooler when it's done in secret. One of the funnest parts of becoming a parent is being able to continue the tradition of keeping (harmless) secrets from mom (or dad).
1. From schnoj:
My wife or I will write notes and put them in our 9 year old son's lunch box most days. One day my wife's note was found by a boy named Max in my son's class and read aloud to his table. Needless to say my son came home quite embarrassed. Since I'm currently unemployed I went to have lunch with my son at school the next day. Towards the end of lunch, he points the boy out to me. We have always preached turning the other cheek, telling the teacher, etc, but something about this kid's face made all those teachings fly out of window. I told my son "now listen, I'm going to tell you something you can say to him but you cannot tell your mother."