They do their jobs well, offering friendly noses and paws to tense passengers in busy airports. However, as with all dogs and humans, a sudden loud noise can startle them into a violent panic attack.

It happened recently in a major airport, when a comfort dog was frightened by aircraft engine backfire. He instinctively clamped his teeth down on a nearby hand at exactly the wrong moment, causing a deep wound requiring a dozen stitches.

So, during your travels and in your neighborhood, when you put a hand out to pet a strange dog, be careful, You may be rewarded with a sudden bite, even if it comes from a formerly friendly canine face.