What does Huck do to Jim that he regrets in chapter 15? Why does he regret it?

The next morning, Huck awakens and luckily manages to catch up with the raft. He finds Jim asleep and wakes him up. Jim is glad to see him, but Huck tries to play a trick on Jim by telling him that the events of the night before were just a dream. After some convincing, Jim starts to interpret the "dream." After some time, Huck finally points out the leaves and debris left from the night before, at which point Jim gets mad at Huck for playing such a mean trick on him. Huck feels terrible about what he did and apologizes to Jim.