I allow my dogs to have an off lead runaround where its safe to do so. If I see an on lead dog approaching I put my dogs on lead (because not to do so may end up with one of them having their faces ripped off by a dog who is on lead because he is not friendly).
I have never encountered a nasty off lead dog until today. I was walking my 18 month and 5 month old dogs through a well known dog walking area (miles of grassland and a river). We met several dogs on lead. Each time I either put my dogs on lead or took them off the path and had them wait (with a treat to their nose) until the other dog had passed. (I am training my 5 month old to ignore other dogs and people). All went well. We met a few friendly, off lead dogs and, as usual, my dogs had a run around with them.
Along comes a bloke with 2 dogs, off lead. As usual I call out to ask if his dogs are friendly. Yes, no problem, he replies. I took this as an opportunity to allow my dogs (the puppy really, as the older dog already knows to pass other dogs unless I give her the OK to play) to greet the dogs, briefly, and move on. My puppy did his usual thing of scampering towards the other dog, to within about 10 feet, and then rolling on his back, in the submissive pose. One of the dog's ran at him and had a go. There was a lot of noise.. With the other dog growling and barking and trampling and snapping at my pup and my puppy screaming.
I had no choice than to get between them and pick the pup up, with the other dog still snarling and growling. His owner made no effort to intervene and, afterwards, said, "I don't know why he did that. He's never done that before". He was concerned about the puppy though and stayed whilst I checked him over. Luckily there were no puncture wounds.
Maybe this was the first time his dog has shown aggression toward another dog. I don't know. But I hope the owner doesn't allow his dog off lead around other dogs again. One show of aggression is enough. If it were my dog I would bring in a behaviourist.
Luckily my puppy wasnt seriously injured. It has bound to have set him back on his socialisation though. So back to square one we go.
PLEASE, if any dog owners do not know how their dog is going to respond to another dog please put it on a lead when passing other dogs. That gives other dog owners a clue that your dog is on a lead for a reason and they will, or should, leash their dogs to pass. It may save ££££'s in vet fees too.