I saw that, not sure if it was posted on here or not. I'm not expert, but I don't think it happens too often.
It doesn't, but it has happened a few times in the last 20 years. Richard Zednik the most recent, about 18 months ago I believe, had his cut by his own team mate (accidentally). He's still playing, but he was out for a good few months.
You get some really bad injuries though, there's a big discussion going on regarding hits to the head as some professionals are suffering recurrent concussions which are forcing them to retire. I believe they're banning it now.
For those of you that just have to see a throat cut though...