Yah, I asked about it. The nerves for your bottom row of teeth and your tongue are very close together so it's very easy to "hit" the group of nerves that goes to both. All dentists try to avoid it, but there's only so much they can do. So when they inject the nerves for the bottom row of teeth they sometimes catch the nerves for the tongue as well. I have no dental training so I'm only relaying information.... in the picture below it looks like spot 20 is right around where it starts splitting .