Treachery is, essentially, a betrayal of trust. Huck Finn is notorious for telling lies with such ease that it is difficult to tell how much of reality he believes himself. Many situations, in Huck's mind, call for fibbing. Sometimes he does it for protection, other times it seems purely for entertainment. He crosses the line into treachery, however, a few times when he lies to the point of losing someone's trust.
One prime example comes in chapter 15, just after he and Jim have been separated on the river by a very dense fog. Once the fog has cleared and the two are reunited, Huck leads Jim to believe that he dreamed the entire night of separation.
Why, de fog!—de fog dat’s been aroun’ all night. En didn’t you whoop, en didn’t I whoop, tell we got mix’ up in de islands en one un us got los’ en t’other one was jis’ as good as los’, ’kase he didn’ know whah he wuz? En didn’t I bust up agin a lot er dem islands en have a terrible time en mos’ git drownded? Now ain’ dat so, boss—ain’t it so? You answer me dat.
Well, this is too many for me, Jim. I hain’t seen no fog, nor no islands, nor no troubles, nor nothing. I been setting here talking with you all night till you went to sleep about ten minutes ago, and I reckon I done the same. You couldn’t a got drunk in that time, so of course you’ve been dreaming.
Huck is playing with Jim here because he realizes how scared Jim was to be separated, and how relieved he seemed to be reunited. In this moment, Huck is purely acting out of a habit of self amusement. What he does not expect is how much he ends up hurting Jim's feelings when he realizes Huck is lying and leading him to believe he is a fool. Jim admits the level of his friendship with Huck in this scene, expressing how scared he was to lose him in the fog, and Huck essentially laughs at Jim's vulnerability. In many ways, much of Huck's survival can be attributed to his ability to lie, but this is one act of treachery that nearly costs him a dear friendship.
Another example of treachery in the story comes in chapter 20 when Huck and Jim meet the Duke and the King. These two characters are professional liars and come with a variety of characters and stories. One of their most common cons is their act as two traveling preachers essentially performing tent revivals and collecting money for "the poor."
Take up a collection for him, take up a collection!" Well, a half a dozen made a jump to do it, but somebody sings out, "Let HIM pass the hat around!" Then everybody said it, the preacher too.
So the king went all through the crowd with his hat swabbing his eyes, and blessing the people and praising them and thanking them for being so good to the poor pirates away off there... (Chapter 20)
The Duke and the King are con artists. They go from river town to river town, assuming the identities of various characters, and con people into paying them money for advice, skills, and causes that are essentially fake. This example of treachery is less personal than the first, and also done, somewhat, out of a motivation for survival. They play on the innocent trust of others, and use that trust to deceive them.