According to Wiki:
A legend exists of his death. During his studies a bookcase fell on him, covering him with books. His students were able to rescue him after an hour or so and he related to them that he had been covered by the books of the authors with whom he had quarreled. He had asked forgiveness from all of them and they all complied save for one, Mordecai Yoffe (known as the Levush) who refused. He knew therefore that he was not long for this world, and pronounced the verse in Hebrew “Aryeh shoag mi loi yiroh”; i.e. that Aryeh (the lion, meaning himself) shoag (roars), but mi (an acronym of Mordecai Yoffeh, but can also mean ‘who’) loi yiroh (is not afraid).[4]