The story, related by the infamous rumormonger Suetonius, may not be true. However, the Emperor Tiberius had, at the time, become isolated and paranoid, so it's also not impossible.
A fisherman appeared unexpectedly and offered him a huge mullet; whereupon in his alarm that the man had clambered up to him from the back of the island over rough and pathless rocks, he had the poor fellow's face scrubbed with the fish. And because in the midst of his torture the man thanked his stars that he had not given the Emperor an enormous crab that he had caught, Tiberius had his face torn with the Crab also.