This was just shared on another dog forum I participate in. I immediately thought of this thread, as it makes it so clear that you MUST be aware and on your toes at all times. Even after you've been a few times! Sorry, a bit long, but I think she wrote it up well:

Dog Park Scary Moments: [ that was HER title in the other forum, not something I put to 'scare' ya!]
I really want to share this story to show the risk of going to a dog park. It's not always a bad thing but I always warn people to use them at their own risk. Feel free to share any moments you may have had at a park.

Rummy is the first dog I have ever owned who I feel comfortable taking to the dog park. He is respectful of other dogs and plays nicely. He will never start a fight and if something does happens he's the first to retreat from the situation. Doesn't have a mean bone in him. Not much of an issue and we go at least a couple times a week. It's a hidden park and usually there are the same dogs we see each time with the same owners. I know there is always a risk each time I go of something going wrong so I am usually always on my guard and am developing a good recall with Rummy to call him off of anything. Today just was not that kind of day.

We walked around the park, usual dogs in the park with a couple I didn't recognize. I let Rummy in, they sniffed each other then I let him off the leash. He ran over to a new dog who was on a leash with a lady. The leashed dog started lashing out towards Rummy trying to attack him, trying to attack the lady. Rummy didn't fight back but cowered down enough that I pulled him away before anything could happen. The lady yelled out "Hold your dog back!". Then she walked out with the dog. A younger kid followed her out.

It turned out the dog was not hers. It was an unsupervised 10 year old boy who came in the park with his friends and didn't know his dog was dog reactive. The dog was a young female Pit Bull. I don't have a problem with Pits but it was obvious this kid didn't know anything about his own dog. The lady was a very experanced dog walker who steped in earlyer to stop another fight with the same dog that had just happened moments before I arrived. I did not see it when I first got there and was unawair of what I had walked into. She spent a good amount of time talking to the kid outside of the fenced in area, telling him not to bring his dog back. Then came back in. I was still shaken up and apploigized for letting my dog approach the dog. She said I did nothing wrong but warned me to watch for something like the hard staring the dog was doing. I regret not seeing that when I steped in. She then filled me in on what had happend, said it is a learning process. No one was hurt thankfully but yikes it could have been worse. I will still go back there but I hope that kid never brings that dog back.