Non-wedding ring on wedding ring finger!

So, I should probably know this, but I don’t.

Is it okay to wear a ring that isn’t a wedding ring/engagement ring on your ring finger?
I saw this girl that had a ring on that finger, but it wasn’t a wedding ring. I thought that was really weird, why didn’t she just put it on her right hand? Is this normal? I should know these things, but I’m super non-observant. It’s bugging the heck out of me.