It happened to me a few times before. It's been a long time since the last one. I didn't freak out because of it though. I guess it's because I already knew I was dreaming.
This did freak me out: While I was half asleep, I would feel like I'm hearing a extremely loud sound at a constant rate. I knew this wasn't true, which freaked me out but, I couldn't shake that feeling and I would try and cover my ears. This would last for 15 minutes or so. It only happened twice in my life; once when I was a kid and another just last year.