The following is quoted from a lecture in the Reformation and Modern Church History series at Covenant Theological Seminary (source).
One story that illustrates the humanness of Calvin deals with his friend, Pierre Viret, who was from Lausanne, where Calvin went to school. Viret was a Reformer in Lausanne. One day Calvin wanted to send a message to Viret, and the way to do that was to ask a couple of students from the Academy in Geneva to take it. Calvin did that. He wrote the message, sealed it up, and then gave it to one of the students to take to Lausanne. Then he had another thought, so he held up the students, wrote another message, and gave it to the other student. When Viret received the two letters, he read the first one, which had to do with the business that Calvin wanted to transact. The second letter said, “Dear Pierre, I do not have anything to add to the first letter. But when I saw that this other student was disappointed that he did not have a letter to carry to Lausanne, I thought I would write another one. Please act very surprised when you read this.”