The scope of the word 'misadventure' is more global than that of a simple prank. I'm guessing that it refers to Tom's (and not Huck's) leading Jim on to think that he still needed to escape to freedom long after the Declaration of Emancipation had been passed. He had been having so much fun in his adventures with runaway Jim that he didn't want this "lark" to come to an end. So he kept Jim "in the dark" for as long as possible:
Chapters 34-43: Jim's Rescue
Tom agrees to help Huck rescue Jim. He insists that the escape follow models from all of his favorite prison stories: he smuggles in items past the unwitting Phelpses. He makes Jim sleep with spiders and rats, and write a prison journal on a shirt. He also warns the Phelpses anonymously. In the escape, Tom gets shot in the leg. Jim and Huck each return and are caught in the act of seeking help for Tom.
Ironically enough, Jim accepts this farce at his expense better than Huck does, probably because he has been conditioned into docility and accepting his fate:
Finally Tom reveals that Jim is in fact no longer a slave: Miss Watson died and set him free in her will. Tom's Aunt Polly arrives and clears up the case of mistaken identity. Huck, upset by the trick played on him and Jim, accepts Tom's explanation that he wanted "the adventure" of the escape. Tom gives Jim forty dollars for his trouble.
Jim is in fact not a very developed character but rather serves as a character foil to Huck:
Jim's primary function is to further the characterization of Huckleberry Finn: by his presence, his personality, his actions, his words, to call forth from Huckleberry Finn a depth of tenderness and moral strength that could not otherwise have been fully and convincingly revealed to the reader.
- from eNotes.com/adventures-of-huckleberry-finn/role-jim-huckleberry-finn
Check out both of the following references for details.
The part you are talking about is in Chapter 10. In that chapter, Huck and Jim have both run away and are on an island in the middle of the river. Huck has found a rattlesnake skin and he knows Jim thinks touching it with his bare hands was bad luck.
Huck kills a rattlesnake and curls it up in a life-like pose on Jim's blanket thinking it will be a good joke. Instead, the snake's mate comes, curls up around it, and bites Jim when he gets in bed.
Jim doesn't know Huck is responsible. He tells Huck that this shows that Huck really did bring bad luck by touching the snake skin.
Huck Finn in "The Adventures of Huckleberry Finn" by Mark Twain is a prankster. He is free and full of mischief. Jim, the slave who escapes with Huck, is very superstitious. Jim is afraid to ponder about a dead man they find. He is also afraid to touch the skin of a snake that has been shed. He believes it causes bad luck. Huck finds a rattle snake and kills it. He thinks it will be funny to curl it up and place it on Jim's blanket.
"I killed him, and curled him up on the foot of Jim's blanket, ever so natural, thinking there'd be some fun. "(49)
The joke backfires because the rattlesnake's mate gets in Jim's blanket and bites Jim. He gets bit on the heel.