Vedius Pollio, notorious for cruelty, once ordered a slave to be thrown alive into a pond as food for the fish because he had broken a goblet.
These fish were moray eels :
"Moray eels' heads are too narrow to create the low pressure most fishes use to swallow prey. Quite possibly because of this, they have a second set of jaws in their throat called pharyngeal jaws, which also possess teeth (like tilapia). When feeding, morays launch these jaws into the mouth, where they grasp prey and transport it into the throat and digestive system."
(source : Wikipedia). This makes being eaten by them a particularly painful death.
It is said that the incident with the goblet happened during a visit of Emperor Augustus, who saved the slave's life by deliberately breaking a goblet himself.
In another version, Augustus bought the slave and then ordered all Pollio's goblets to be broken.
A third version I once read, was that Vedius Pollio served eels for dinner. They were very delicious, and Augustus wanted to know how he did it. Vedius Pollio simply answered that he regularly fed his eels with a slave. Augustus was so disgusted that he ordered Vedius Pollio to be thrown into the eels' pool and getting eaten by them.