The first mate was found to be drunk one day and that day it happened to be the captain's turn to write in the ship's log so he wrote :
The first mate was drunk today.
He begged and pleaded to the captain to remove that entry but the captain argued that once an entry was made in the company's log it couldn't be deleted. The first mate decided to get even.
The next time it when it was the first mate's turn to write in the log, he wrote:
The captain was sober today.