It happens all the time, two or more dogs greet each other and their owners misinterpret the signs the dogs are displaying. There is some posturing, maybe some teeth bearing but, some tail wagging so the owners think everything is okay. What people don't realize is that there are different types of tail wagging. Let's look at tail wagging a littler closer by looking at the situation and the rest of the dog's body language. Of course, it all depends on the individual dog.
Usually, a happy wag is a low, slower, wide wag, usually but not always. Look at your dog's wagging tail the next time he sees you approaching, obviously, that's his happy wag. Now, if you have, or know, a dog that gets nervous, aggressive or barks at strangers, look at that wag. Chances are, his tail will be straight up, maybe pointing forward and moving back and forth very quickly. He may also have hackles, partial or full, his ears may be up and he may be growling or baring teeth. These are not the signs of a happy dog.
There are some pugs in our neighborhood that Cooper absolutely loathes. He will sit in the window, happily watching the street when suddenly, he will stand up, ears go up, hackles rise and his tail gets erect, goes forward and rapidly whips back and forth. Sure enough, there they are. It's absolutely classic.
So, remember, the next time you're trying to assess a dog's comfort level, don't assume everything is okay because her tail is wagging. There could be pugs out there!