Nurse sharks can cause significant tissue damage and have a reputation for being extremely persistent and also for not releasing after they bite something.
This is second hand info, but I had good friends who reported that their female neighbor was diving in a swim suit and suffered an unprovoked attack and required skin graph(s) on her butt. The particular shark had developed an aggressive behavior (for some reason) and had gone after other divers previously.
Nurse sharks are generally not a problem if you don't mess with them (or feed them), but they are not something to trivialize.
I don't trivialize them and though amused the time they swam bumping my legs, I was not amused in the least when the nurse shark ducked my finger. I was extremely grateful that it realized its mistake so quickly.
Neither was I amused the time that I found myself completely straddle a very large nurseshark on a night dive. I was ever so grateful that it seemed not to even notice that I was there at all. Couldn't get myself free fast enough.
Though I'll admit that it has made for a very entertaining story since then.. yeah, ride 'em