Ray Ortlund relates this story:
Are we to continue in sin, that grace may abound? (Romans 6:1)
Johann Tetzel was a Dominican monk who went around sixteenth-century Germany selling indulgences. He used a little song: "As soon as the coin in the coffer rings, the soul from purgatory springs!" So a thief came up to him one day and asked how much it would cost for an indulgence to cover all his past sins. "A thousand gold pieces." "And how much to forgive all my future sins?" "Two thousand more." "All right, here's three thousand. Give me the indulgence." "Here it is. Thank you." "Now here's one of those future sins. See this knife? Hand back the three thousand."
Ray explains the grace in Romans 6 masterfully here.