The first mate was found to be drunk one day, and that day happened to be the captain's turn to write in the ship's log, so the captain wrote “The first mate was drunk today.”

The first mate 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 was his responsibility to write in the log, he wrote “The captain was sober today.”