I wandered into a bereavement group for spouses once while looking for another group. I was there a good 15 minutes before I figured it out and then knowing I would be late for my group, I just jumped up and said "I'm so sorry for your loss everyone but I'm not supposed to be here."
One sweet lady said "yes, honey you are so young to be going through this!" Then I was really embarrassed and told them I had the wrong room.
And then the same lady says "well, when it happens we're here for you!!!" I just started laughing. I could not stop. I don't know why, I felt like an idiot.
So in answer to your question, I am sure it happens a lot.