My wife and I were playing Tennis. We're both pretty new to it and knew we were supposed to hit the ball back and forth but couldn't remember the finer details of scoring. I thought that it was best of 5 sets.  She thought it was best of 3 sets.

We learned later that Men's professional is best of 5, and Women's Pro is best of 3. Go figure

There's actually a valuable lesson to be learned here. Discerning that lesson will be left as an exercise to the reader.