When I look at my current 2 - they were both taken along to the local dog park at around 16 weeks for socialisation. With Lola on her first outing, I misjudged a dog and he went for her, I have never forgiven myself for that and was careful with her from then on, however there were 2 other occasions where she was really frightened. Her easy going, fantastic nature has prevailed, however her attacks were from large breed dogs and to this day she is not very comfortable around larger dogs - little dogs she loves. Harry didn't have any issues with other dogs, however imo he grew a little too sure or himself and dominant, almost like a bully too young.
My next GSDs, will I take them to the dog park? NO, I don't think I will, as someone else said, I will only allow them around really stable older dogs.
I have seen many a puppy come into a dog park and attacked. When I think about these situations, both owners (puppy owner and dog attacking owner) were at fault. The puppy owners are nervous and this impacts on the puppy and the attacking dog picked up on the puppies instability through nervousness.
My previous GSDs were not taken to dog parks and I don't recall any instability with them.