Nobody has ever deliberately brought a TRULY aggressive dog into our dog parks. HOWEVER, certain dogs (rambunctious adolescent Sporting breeds come to mind) can possess lackluster manners. When placed in a large, social setting, pack mentality kicks in. And when a dog snaps or growls at a rambunctious adolescent to warn it, that snap can initiate every dog around it. The same thing happens when young puppies are brought into the big dog areas. The puppies yip, the other dogs flood the puppy, and a mass attack ensues. It's disturbingly common.

Also, Kokopup, I believe the OP is referring to off-leash dog parks, where the dogs run uninhibited and socialize freely. Do Dah Day, while it looks very fun, seems like a completely on-leash parade VS. a traditional dog park.