What did George do once that made him stop playing jokes on Lennie in Of Mice and Men?

One day George told Lennie to jump into the Sacramento River, assuming Lennie knew how to swim. Lennie did jump, but he did not know how to swim, so he nearly drowned before the frightened George and other men could rescue him. After this experience of nearly causing Lennie's death, George has played no more practical jokes on him.
In Chapter 3, George is in the bunkhouse and talks with Slim, who remarks on Lennie's strength and his ability to do so much work. "There ain't nobody that can keep up with him," Slim declares. Then, Slim observes that George and Lennie make an odd pair, inviting George to talk about himself and Lennie. George tells Slim that they are from the same town and Lennie was cared for by his aunt. After Lennie's Aunt Clara died, he started to work with George. "Got kinda used to each other after a while," George remarks.
George then confesses that he became accustomed to playing jokes on Lennie because he was "too dumb to take care of himself" and it made George seem smarter. George even admits that he could be abusive to Lennie, and the big man would not even become angry. "That wasn't so damn much fun after a while" (Chapter 3). Finally, George explains that he stopped playing practical jokes on Lennie after the time he ordered him to jump into the Sacramento River and Lennie jumped without even knowing how to swim. Because Lennie nearly drowned before being rescued, and he had forgotten that George was the one who told him to jump, George felt so guilty about this trick that he stopped his pranks against Lennie, saying, "Well, I ain't done nothing like that no more" (Chapter 3).