Never walk past a mistake.
This is one of the first lessons drilled into young military leaders.
To put it another way: make-on-the-spot corrections. This serves a number of purposes. First, and most obviously, correcting a mistake shows attention to detail and reinforces standards within an organization.
Second, it teaches aspiring leaders to have the moral courage to speak out when standards are not being met. You never look the other way and pretend you didn't see it just to avoid a confrontation or to be seen as petty.
Third, it shows the followers that you care about them, the unit, and its mission. If a follow knows that he has just made a mistake and gotten away with it, he loses confidence in the competence of the leader and has less respect for him.
Fourth, you set the example for all of your subordinate leaders to act in the same manner. High standards and mutual respect will flow up and down the organization.
Fifth, it keeps mistakes and screw-ups from moving to another level or, even worse, propagating. Take care of it now. Don't assume somebody will take care of it later...even if it's their responsibility.
From "It Worked For Me" by Colin Powell