My GF has a bulldog/mastiff mix. Fuckin' huge ass dog. We were walking him around, sweetest thing ever imaginable.
A few hours later, we were sitting at the table, and he comes up to me, before i could go to pet him as usual, he lunges right at my neck. I dodged back and he caught a small bit of flesh, nothing dire. Thinking i did something to piss him off, i left him alone for a bit. a few hours later, the same thing, came up to me calm, and suddenly went for my neck again. We put him outside. From that point on, he would growl at me whenever he saw me. He even snapped at my GF and her parents a few times. He seemed to go off at the most random moments.
Eventually they got him neutered and hired a trainer. He seems to be doing much better and seemingly doesnt want to murder me at will. I am comfortable around him, but will ALWAYS be cautious around him. He is pretty sweet (and incredibly clumsy) but i will never forget his behavior, and have a reminder on my neck.