This is what I found on the internet about nurse sharks:
Nurse sharks are often regarded as harmless to people, but there have been cases of attack, some unexplained, but others provoked by spearing the shark, pulling its tail, kicking or trying to take a ride on it. The attacks have not been fatal but in at least one case the shark's vice-like jaws had to be pried apart with a tool to free the hapless victim.