Continuity mistake: In chapter 17, where Huck first meets the Grangerford family, Col. Grangerford is described as "grey and about sixty". However, in the next chapter, the colonel is described in more detail, and his grey hairs are gone. Huck describes him as "clean-shaved, all over his thin face", and with hair "black and straight".
Plot hole: Virtually all editions of the book have been abridged by removing one chapter of the section during which Huck and Jim are rafting down the river (this chapter is known as "The Raftsmen Passage"). While the chapter in question does indeed slow the story down, it also reveals several key points of information, notably Huck and Jim's location on the river. In the following chapter, Huck is aware of this information. With the "Raftsmen Passage" removed, he would have had to have picked it up by telepathy, as there is no mention of him leaving the raft or otherwise having any source for his information.