Drop the ball
Definition 1 of 2
Pro Tip 1/3Expectation Failure
Used when someone fails to meet what is expected of them. 
In the championship game, the goalkeeper dropped the ball.
Pro Tip 2/3Task Responsibility
Refers to missing a personal responsibility or specific task. 
As team leader, you can't afford to drop the ball on this project.
Pro Tip 3/3Sports Origin
Originates from when a player accidentally drops a ball in sports, leading to a mistake. 
He was a great player, but he dropped the ball during the final inning.