"The Godfather" Marlon Brando, Lenny Montana 1972 Paramount Pictures
The mistake: Luca Brasi was Vito's most valuable soldier, but he wasn't exactly the brightest bulb in the box. So why send him in a mission that merely consists of collecting information? And pretending to betray the Don while everybody knew his loyalty was unshakable. No wonder the Turk thought the Don was slipping.
The consequence: Luca Brasi sleeps with the fishes, and Vito ends up concluding the deal, it only cost him the life of a son, and the promising (law-abiding) career of another.
Alternative: If his Sicilian instinct (rightfully) warned him about an upcoming danger, then he should have made Luca a full-time bodyguard and send someone else go spy on the enemy.