Nobody has ever deliberately brought a TRULY aggressive dog into our dog parks.
I guess everyone has been pretty lucky with dog parks. I have been to seminars (for aggresion issues and rehabilitated it), and there was quite a few people that actually came out and said that they brought their dogs to the dog part to fix the problem. Luckily they finally decided that the dog park is not going to fix there dogs aggresion and decided to actually go to a trainer. I know from me watching dogs in the dog park, I have never seen any aggresion, but I have seen some dog fights and unsocilized dogs.