Picture this. The very best employee you've ever had just stormed into your office threatening, "If you don't do something about Mr./Ms. Nightmare, I'm out of here! They've messed up another job, and I'm the one who had to take the heat. You need to do something!"
As you listen, you think to yourself, ‘I'm short-staffed and the workload is going through the roof. I've got to figure out how to get everything done yesterday, and now this. What do they think I am - magic?' Of course, your actual response to this overwrought employee is that you'll take care of the matter immediately.