It isn't fatal (assuming of course you don't get stuck in the heart) but it is extremely painful. It hasn't happened to me but I've read about it. It blisters up really bad and even after it heals you will experience pain in the area for a long time. The area of the sting may later become permanantly numb. Most emergency rooms around here don't know how to treat the sting because stingrays aren't native to this area. But, apparently soaking the sting in hot water is the best method for pain relief. Antiobiotics could possibly be needed to prevent infection. I think theres a sticky about this, but I could be wrong.