First, here's what not to do:
1. Don't hope that if you act like it wasn't a big deal, your boss might think it wasn't a big deal either. This strategy will actually compound the damage: Your boss will be far more alarmed that you don't really care that you made a mistake than she will be by the mistake itself. Rather than making the mistake less noticeable, what will really stand out is that you're not taking responsibility for it.
2. Don't try to cover it up. If your boss finds out later on that you hid a problem from her, you will have a permanent credibility problem—as well as a boss who may feel compelled to poke around to find out what else you might not be telling her about.
3. Don't make excuses or be defensive. This almost guarantees your boss will respond badly.
So how do you handle it well when you've botched something? First, you need to know what happens in your manager's head when you make a mistake. Beyond thinking about the repercussions of the mistake itself, she's worrying about what it means for the larger picture: Did the mistake happen because of sloppy work habits or was this one isolated incident? Is there a fundamental problem with your systems or approach to the work? Do you "get" that this is a big deal, or are you shrugging it off and thus likely to let something similar happen in the future?
Once you understand this, the formula for handling a mistake well becomes more intuitive:
1. Tell your boss what happened, immediately. Don't put it off out of fear. She'll be more upset if you put it off, and delaying sends the message that you value your own comfort over the needs of the work.
2. Take responsibility. Use words like: "I really screwed this up. I'm sorry." In fact, the more concerned you seem, the less likely she is to feel she needs to impress the severity on you. After all, if you proactively show that you get it, there's no need for her to underscore it.
3. Tell her how it happened and how you plan to ensure it doesn't happen again. It's not so much that she wants to know as it is that she wants to know that you know. If you don't understand how it happened, you're not well equipped to keep it from happening again.
You're going to make mistakes from time to time, and any halfway human boss knows that. As long as your mistakes remain occasional and not constant, what matters most is how you handle them.
Alison Green is chief of staff for a medium-size nonprofit where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. She is working with the Management Center to coauthor a book on nonprofit management. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.