Dear Bark: I was bitten by a friend’s dog, which shocked us all because the dog was wagging her tail. What’s up with a friendly dog who bites?
This is a great question, and one that I hear frequently in one form or another from clients. Just like you, many people have been bitten by a dog they thought was friendly because the dog was wagging her tail. This common misconception—that a tail wag is a sure sign of friendliness—creates risky situations in which people believe they are safe when they actually might not be.
The confusion arises because, while friendly dogs do tend to wag their tails, they are not the only ones doing the wagging. Lots of dogs wag their tails when their intentions are anything but friendly. Tail wags are far more complicated than you might think.
via Whisker Therapy