My dog was doing well until a "trained" dog, off the leash, attacked her. Wounded face, badly bruised ribs.
Then, about a year later, a "trained" dog, off the leash, ran up to her and went to sniff her, she took it as a threat, and another fight happened. No blood this time, but still.
Then about a year later... a "trained" dog, off the leash... same fucking story.
10 years later, I am more vigilant and proactive. If another "trained" dog is around (telltale because of the lackadaisical behavior of the owners), I get my dog right up to heel on the other side of my body and get ready.
At least 10 times over those 10 years, I have a dog running at my dog with an owner just standing there yelling "don't worry, they're nice!", me replying "MINE ISNT", and then them jogging leisurely over while I am hunched over trying to keep the 2 apart.
I dont care how well you think you've trained your dog, be a responsible owner. If it isn't your home (front yard NOT included) or a dog park, leash. Also, don't approach a dog you don't know. You don't know their history. Be aware of your surroundings and pick a route that keeps your dog leash distance from the other dog.