Lungfish have such an efficient and nasty bite that they can remove a finger before you realize there's been a bite.
Here's something to give you an idea of the force of their bite. I temporarily housed a 12" and 15" mbu puffer in with a 20" lungfish (you know how leathery and tough puffer skin is). The lung managed to kill, crush, and eat the 12" mbu in the span of time it took to finish maintenance on the mbu tank.
Another; I used to feed another lung full grown Louisiana red crayfish. You could here the lung crunching up the cray from across the house.
So, a nice soft finger would easily become a lungfish snack.