Everybody screws up. I screw up. You screw up. Even the Queen of England screws up. Shoot.
When someone who works for you screws up, there are two things that can happen. Either the employee realizes he screwed up, or he doesn't. Either way, your job as a manager is to notice the screw up and correct it, either by doing something or - and this is sometimes harder - by doing nothing.
Depending on the person making the mistake, there are a range of possible behaviors and things you have to do about the situation.
Sometimes the person has no idea he's screwed up. Seriously no clue that a mistake was made at all or that he caused it.
Reaction: If you don't point out the mistake, he'll repeat it eventually. However, what you really want is for the person to start noticing his own mistakes. So don't point it out directly; instead lead him down a path so that he comes to his own realization that he made a mistake.
Sample Phrase: "I noticed that you didn't clean up the machines when you were done with the test. Were you still using them?"
Hide and Seek
Sometimes the person recognizes that there was a mistake and tries to cover it up.
Reaction: If this person hides the little mistakes, the big ones will only blow up on you later. You have to point out the mistake, yes, but the more important aspect is making sure the person understands and - and this is important - feels safe enough to acknowledge mistakes. Be extremely gentle with the mistake itself, at least the first couple of times.
Sample Phrase: "I noticed that the memory tester was broken in half under the keyboard in the QA lab. We can get it fixed, but you need to come tell me that it's broken so we don't have to go hunting for it."
Sometimes nothing you can say is worse than what the person who made the mistake is already thinking. Left alone, this person will beat himself up for every screw up for longer than you might.
Reaction: Punishment is already happening; your reaction needs to be about acknowledgement, correction, and speedy moving on.
Sample Phrase: "Yup, you that bug is a duplicate. Let's figure out how we can make searching the defect tracking system easier in this case. [do so] Okay, now we've improved our process. What's next?"
Sometimes the person acknowledges the mistake, corrects it, and moves on to the next thing.
Reaction: This person screwed up and dealt with it. Anything you do will prolong the incident unnecessarily.
Sample Phrase: None.
When you're working hard, and you're tackling challenges, mistakes will happen along with triumphs. Making mistakes okay - both to make them and to deal with them - is just one of the things you need to do to keep your team taking risks and getting better at what they do.