You can't teach an old dog new tricks. Perhaps unfair to geriatric canines, but quite applicable to the workplace.
I've gone through a heck of a lot of management training and theory, and that one adage probably explained more to me about management than years of study.
The simple fact is that people can't change what they are. If an employee is impatient, she'll always be impatient. If an employee is sloppy, he'll always be sloppy. A poor manager complains about this fact. A good manager uses it to her advantage.
It's like the old joke about heaven and hell. Heaven is where the French are the cooks, the Italians are the lovers, the British are the cops, and the Germans are the engineers. Hell is where the British are the cooks, the Germans are the lovers, the Italians are the cops, and the French are the engineers. The point is, know what people are good at, and don't expect your German engineer to be a love god. That's just not who he is. (Pop quiz: Name one modern German sex symbol)
I had to fire people who would have been perfectly capable employees, were the circumstances different. But a methodical bureaucrat should't work for a startup, and an impatient cowboy shouldn't work for General Motors.
Every employee (even you, you budding capitalists) is imperfect. Just make sure that his role emphasizes his strengths and hides his weaknesses.
By the way, this principle also helps prevent one of the most common management mistakes: spending time improving poor performers and neglecting your stars. Face reality: Your dead wood should be fed to the chipper so that you can spend your time working with the top talents in your organization.
And for your employees out there: if you choose your role carefully, you can avoid being Fargoed like Buscemi.