Has anyone ever had a cavity? What happens then?

I've only ever had one cavity and it was in a baby tooth. But once it was filled, the tooth was fine for over a year and the only reason it wasn't after that was that it fell out. My mom had fillings that were fine for 25 years and then she had problems, but that's mostly attributed to the fact that her orthodontist's cement for her braces was defective and ruined her teeth.