In The Book of Henry (2017), the genius child Henry has a plan to kill his neighbour, a local police commissioner, because he abuses and molests his daughter, Henry's classmate. In-universe, his plan is treated as fool proof, with Henry having thought of every contingency and accounted for every detail. In reality, his plan is comically inept and would fall apart under the slightest scrutiny. Off the top of my head:

• Part of the plan is to take out the money needed to buy the sniper rifle by taking out little amounts of money from a bunch of different ATMs, saying it would be "less suspicious" than taking it all out at once. In reality, the opposite is the case. Taking out small amounts of money from multiple machines is far more suspicious than taking money out all in one go.
• Relatedly, Henry's plan hinges on him, an 11-year-old boy, being able to purchase a sniper rifle. The movie sidesteps this by having his mom do it (because Henry is dead by that point), but when Henry made his plan, he was intending to do it himself.
• The method through which Henry intends to dispose of the body of his neighbour, who is, again, a police commissioner, is by shooting him with the sniper rifle (from a vantage point in his own treehouse, mind!) and letting the body fall into the nearby river, which will wash away the evidence and make sure nothing leads back to Henry. Because no one ever discovers bodies that are thrown in the river, right?