Because we so often caution people about possible issues when frequenting dog parks I decided to post this excerpt from an article about socialization because of its discussion of using dog parks. It might lend support in future when the issue comes up again, also because of its identifying dog parks as potentially problematic for any type of dog, not just border collies.
Dog parks
Even though they're called DOG parks, they're not a good option for every dog, and despite conventional wisdom that says the best place to socialize a dog is at the dog park, they're an exceptionally bad