Full disclosure I love both breeds and did not personally see this. Glad I didn't. I had a customer Ed who trained police dogs part time with the local departments in WA and owned nothing but mastiffs. My limited experience with mastiffs is they are giant love bugs with people. Story goes he was onsite with a local PD doing some dog training with his mastiff just along for the day and a police dog German Shepard kept harassing his mastiff by barking and biting at it. When it finally had enough and without a sound the mastiff whirled and bit the Shepard's head clean off in a blink. Given their size and power I tend to believe it. Amazing critters.