It's never happened at my gym, and I've been there over 4 years now. We have had guys get wobbled, but never lights out. I've had the lights 'flicker' twice myself, but never been out. Both times the guys I was sparring instantly let up and asked if I was ok. When it happened the few other times with other guys, their sparring partners stopped too.

There really is no reason to get KO'd like that in training. It shows a lack of control people sparring should not have.