I like to walk my dogs in a local nature preserve. Despite the clearly posted leash laws at every entrance, people still like to walk their dogs off-leash here. This is obnoxious at beast, and dangerous at worst. Most of these people have no control over their dogs, who love to harass/annoy/fight leashed dogs. I carry various methods of warding off larger and potentially dangerous dogs, and I normally ask people to call their dogs or inform them that I am not friendly.
While I do everything possible to prevent larger dogs from making contact with mine, I sometimes let the little ones take their chances.
Sometimes it's really muddy out there.
I try to warn them, but even "My dog will eat your dog!" doesn't seem to get the message across. I sort of hope that arriving with a white dog and leaving with a brown one will help them see one of the many benefits of leashes, but that's probably too much to ask.