When I play tug with Layla, she growls and carries on like a maniac. We've always called her "psycho puppy" when she starts growling like this. It's all in play, though.
I was just playing like this with a stuffed toy of hers (then throwing it down the hall for her) and Grim heard her growling and saw that she was engaged with me (the stuffie was small... I don't know if he thought she was biting me or what) and he came running up and grabbed her by the scruff and pulled her away from me. He was NOT playing.
When he got her off me, he moved to stand in front of me, blocking her. It wasn't resource guarding as he doesn't care if I pet her, etc. That's not the issue. I've NEVER seen him react to her like that. They are best buddies. He didn't growl at her or bark or anything... just pulled her off and then 'stood guard' over me. I know one thing now... if any dog comes at me, Grim will try to take them out.