I've experienced SP several times. I never had an accidental feeling of it through regular sleep so my first contact with it was after learning the WILD technique.
I knew what it was, how it would happen, I just didn't know what it would feel like. And I must say the feeling you get is pretty novel and unique. I perfectly understand that it could be terrifying if you didn't know what it was. My thoughs were more along the line of 'Wow, this is freaking weird...yet wonderfully interesting'