My dog, Charlie, sometimes has a problem with not knowing the difference between a playful bite and a painful one. We play around a lot and she nibbles on my arm softly. Sometimes, though, she gets way to excited and it starts to hurt. I pull my arm out of her mouth and I use a tone to make sure she knows that what she did was not okay. Then goes back to playful nibbling!
There was one time that she bit my house help. Charlie has a sharing problem which I'm trying to get her to kick. At times she plays around with you but there was one time, Charlie had a bone and she dropped it. My helper picked it up to give it back to her but I think Charlie thought she was trying to take it away from her and bit her 3 times. It never happened again after that. But sometimes we do worry whenever takes off with one of our slippers or shirts and we try to take it, because she starts to growl.