When I was younger, I had a commercial-grade stapler (the big ones that advertise stapling through N-hundred sheets at once) go into my pointer finger on my left hand. The resulting bone infection eroded all the bone in my fingertip, which was reconstructed later from "donor" bone.

Step one of the operation was getting taken back to a patient room, being given an IV (since I was not to eat or drink for 12hours before the operation), and waiting. After a few hours, a nurse walked in with a sharpie and wrote "NO" on my other hand.

It wasn't comforting.

It happens. http://www.nytimes.com/1995/09/17/us/doctor-who-cut-off-wron...

Coincidentally, two posts talking about this kind of never event, with links:

https://news.ycombinator.com/item?id=12773709

They do happen, but they're quite rare.