Most BPs do warning strikes. I mostly only get it from babies in shed, and it's usually because the dog was poking her nose in to take a look.
Most of the bites I get are also from babies/yearlings, but those are generally feeding errors. Plenty of warning strikes are open-mouth, but they use very little teeth compared to feeding errors. A lot of times I just get gummed and drooled on.
....and yeah, I've had exceptions. Mr. Nibbles and Nancy, the Pewter Who Knows How to Make it Hurt. They both tagged me non-stop for their first 12-18 months. After a while, I swear it wasn't defensive: Mr. Nibbles was tagging me for fun, and Nancy out of sheer spite. I had to walk away from both of them several times because they made me so angry.
Now I'm breeding Mr. Nibbles and Nancy together to make pewter het pieds, and I'm kinda worried about the evil little monsters I might be making...