They didn't go away this time, so I gave my usual spiel about how Galen apparently missed some socialization as a puppy and therefore gets rude sometimes. She said her dog was pretty unoffendable, so we let them say hi. That went fine, and she found a tennis ball (actually, I pointed it out after she mentioned that's what her dog was looking for in the bushes -- the field's right next to the tennis courts, so there's usually a lost ball or two around somewhere) and started throwing it for the dogs. They both chased after it for a while -- maybe 5 or 10 minutes -- and seemed to be having a good time with that.
Then quite suddenly they were fighting. The other owner said she thought her dog growled and Galen took exception to it; about all I could see was that they arrived at the ball at about the same time and suddenly there was snarling and biting. We broke it up quickly, and checked the dogs for injuries. The only thing we could see at the time was a scratch across the other dog's muzzle, so they left one way and we left the other way. But on the way out, I noticed that Galen (a) was walking oddly and (b) had managed to knock out one of his teeth. So when we got home (which took a while -- he insisted on lying down a few times on the way) I called the vet.
The odd walk went away within a few hours, but Galen spent the whole day and very nearly a thousand dollars at the vet today, getting his jaw x-rayed and the broken teeth removed and everything else cleaned and the broken canine he came with sealed to prevent infection and on and on and on. In addition to the visibly missing tooth, the two teeth on either side of it were also damaged badly enough that they had to be removed, along with the remaining root bits from the first one. Fortunately, these are all incisors, the very small teeth along the bottom front of the jaw, which dogs don't actually use for much. So he shouldn't have any trouble eating or anything once he heals up.
He is restricted from chewing hard things (including rope, which means not his favorite tug toys, or tennis balls, which really knocks down the fun-things-to-do list) for a month, which is a long time. Right now he's groggy from the anesthesia and generally unhappy with the world. We have kibble soaking in chicken broth to soften it up for this evening's dinner; if he doesn't want that we may get a can or two of soft food tomorrow. (Added later: Doesn't want it. Had a few spoonfuls of yogurt instead.)
I feel terrible. I really hope the other dog got out of it with just the scratch we saw there, and not any harder-to-spot injuries.
Operation Find Nice Confident Dogs to Play With is on hold until we figure out what the heck happened there and how to make it not happen again -- we've had the initial introduction not go well, and we've had Galen playing more roughly (wrestling) or intently (chasing) than the other dog was comfortable with, but we haven't had playing well go bad before and I didn't see any signs of a problem before there was a big damn problem.
There were some caution flags -- things that sound like potential problems, based on what I knew going in. The other dog was large (a Lab), and bigger dogs make Galen more nervous than smaller dogs. Running after a tennis ball is going to engage prey drive in a way that just running around with another dog perhaps isn't. A tennis ball is a resource -- a scare resource, given one ball and two dogs. A resource that the other dog was known to be particularly fond of, even. I feel like I should have seen it coming, even though all the body language looked fine until the fight started. There are a bunch of things I could have done differently: not let them play at all; done introductions on-leash and kept them quietly near each other for a while first; asked the other owner not to throw the ball; added another ball to reduce the competition; enforced some rest breaks.
I tend to defer to the other owner when Galen's interacting with a dog, on the theory that they've probably had the dog longer and are therefore better at reading its body language and knowing what sorts of things are likely to be a problem, and I need to stop doing that. It takes me a while to think through whether something is likely to be a good idea, socially, and that includes with dogs. I need to install an interrupt so that if the other owner starts doing something I haven't thought through I can ask them to stop until I've considered it. Any ideas for a default phrasing I can use to quickly interrupt an owner doing flabbergasting things like encouraging her dog to chase a squirrel that was outside the boundaries of the off-leash area? (That's two things I don't want Galen doing -- chasing squirrels [I allow this, if he's not supposed to be doing something else, but I'd rather he be so used to ignoring them that he just keeps on ignoring them, so I don't want anybody encouraging him to look for far-away squirrels peacefully minding their own business] and leaving the off-leash area without me [absolutely not on].) I was too stunned to figure out anything to say about that, but I bet it did not help at all in terms of getting the dogs riled up and closer to trouble. I need some kind of stock phrase to throw out while my brain is still going "What? Why would you want your dog to chase squirrels he hasn't even noticed yet?"
And hugs. I could use some hugs.