"Swings and roundabouts" - Apparently it's in reference to playground stuff. I always thought it was "Swings in Circles" but I was wrong. Most people I know use it to mean "what goes around comes around", but that isn't its true meaning. It means that there are as many advantages as disadvantages, so no loss or gain from a situation.
Idioms have always fascinated me, because I take everything literally first. So whenever I hear people use them, or if I hear a new one, it's a silver lining to my day, and a reminder of my autism. I like to query them, and ask people about them, but I'm always met with strange looks as people think "why is it such a big deal?".