My personal favorite is when Huck arrives at Aunt Sally's house and gets stuck playing the part of Tom. Although he's been lying about stuff for most of the novel, this is the one time he actually knew a little bit about what he's saying. Of course, the problem is caused when the real Tom shows up and has to pretend to be Sid Sawyer.
Another good one, if you're into morals, is Huck playing the trick on Jim after the raft and canoe were separated in the storm. Huck thinks he's just lying to play a joke on a naive slave, but it turns out that Jim isn't as simple as Huck thought. Learning that Jim was genuinely concerned about Huck makes his internal conflict that much more difficult to bear.