Sometimes when braking ,the car slows down but keeps rolling,

What is your question?

When this happens , I quickly put it in park. A few days ago I had a tune up and everything on my car. The mechanic couldn't find anything wrong with the brakes. When they drove it it was okay. Then after I left them, I was stopping at a light and it kept rolling so I had to go over the line and over a railroad track to stop it quickly. It happened again on the way home