Continuity mistake: Tom and Huck have just swore they won't tell about the murder. Tom writes an oath, and Tom and Huck slit their fingers to sign it in blood. Tom, slits the index finger on his right hand. He signs the oath, however, with his index finger on his LEFT hand.
Plot hole: When Injun Joe is about to kill Huck, Tom takes notice of the chest full of gold coins and then empties the chest out (offscreen) because it was too heavy for him to lift, and then tricks Injun Joe into trying to retrieve the empty chest, resulting in Injun Joe falling to his death. The amount of time that passes between Tom glancing at the chest and then tricking Injun Joe before he can kill Huck is roughly 5 seconds. There's no way Tom could have emptied out of the chest in that little amount of time if the chest was too heavy for him to lift in the first place, especially without making a sound.
