We have taught him bit inhibition since he was 8 weeks old. He is normally VERY good at it. We say "easy" when he comes to bite and he doesn't bite hard.
He's teething now, and when he gets excited he sometimes forgets.
Last night he bit my girlfriend really hard, punctured her skin in three places, and scared her pretty bad. She was playing with him. He got really excited, and then she was ready to quit and stopped responding to him bringing the ball. He wasn't having it and insisted several times via nips that he wanted to continue. I've never scruffed and submitted him, but my emotions got the best of me this time unfortunately. While he still had her hand in his mouth I leaped from the couch and yelled "No" and scruffed him. He whelped and wasn't happy about it. In hindsight, I feel bad that I did that.
This particular bite was more an aggressive bite than an "I want to play" bite. He's just about 38-45 lbs now and 14 weeks roughly, so I'm concerned that if I don't show him that isn't acceptable with some force it will continue. When he is a 100 lb dog, which he's on track to be he will really hurt her.
**EDIT** He is also a singleton from a first time bred dame. Having no brothers and sisters probably doesn't help the biting, but he does VERY well with bite inhibition training. We practice it every second of the day. I've even bitten him back when he was a younger pup (10 weeks or so) and I play with him like a dog would. We're enrolling him in puppy training in two weeks at 16 weeks.
What do you guys think of this?