I've never had one, but a friend of mine had a bad experience from meeting someone online. She met him in a public place, at the mall, and they walked around, did some shopping, ate at the food court. It was all fine, except that they just didn't click like that, she liked him but only as a friend. He got mad when she told him this and he keyed her car.
Obviously though, she was there, standing right in front of him when he went off on her and did it, so she did press charges and it was repaired, but it could've been so much worse if they weren't in a public place I think. If he really had the chance he might've done worse for all we know!
But then again, she also met him only a couple days after they began talking on OkCupid