though I do agree, I dont think this is an absolute certainty. When I personally play with my dog we wrestle around and bite (put mouth around and let go) of each others paws and hands, as well as ears and neck and such. She completely understands this is play. Even when she is all hyped up my hands will be in her mouth non stop yet she never puts any force down at all. She is also absolutely aware that she can only do this with me. My gf can put her hands in my dogs mouth, intentionally or during play and my dog instantly refuses them. Same with my nephew...or anyone really. She is aware, like you said, that "biting" is a play thing and can only be done with me or other dogs.

I do agree though, if you can avoid this happening at all, do so. Its not something you want to willingly teach a dog. I just allow it with me because when her and I play, we play extremely similar to how two dogs would. Its not worth taking the risk if you dont know your dog.