Fact: You are going to screw up something at some point in the near future. It's unavoidable, but it isn't exactly something to fret about either. No one is perfect and everyone makes mistakes. The good news is that very rarely are these mistakes something that will make or break your future or even drastically alter someone's opinion of you. The bad news is that the way you react to and handle those mistakes may actually be much more important than you think.
Scenario: You're working as an intern and your boss sends you on an errand to buy some more soap for the office bathrooms. You're not very happy about this because this isn't what you signed up for and you're thinking you deserve some better opportunities. You go to a local store, purchase the soap as requested, and return to the office to deliver it to your boss. One little problem: you forgot to get a receipt as your boss requested, he is somewhat mad, and asks you how he is supposed to properly expense this purchase when you can't even remember to get a receipt.
A: Ignore the question and sulk at your desk while making a sly remark about how you're too good for this and maybe if he got his own receipt he wouldn't have this problem.
B: Apologize, admit your fault, and offer to return to the store and ask for a receipt or pay for it out of your own pocket. Thank your boss and promise to learn from this mistake and never repeat it.
It's probably pretty clear that the right response is some variation on B, but it isn't just about making nice and trying to fix what you did wrong. The correct way to handle any mistake you've made or trouble you find yourself in is to be mature enough to realize that you are probably, at least to some degree, at fault for what went wrong and to learn to accept that responsibility with grace. The point, in this example or any similar situation, is not simply to try to fix things, but to demonstrate a certain level of poise.
It may seem counterintuitive, but sometimes admitting and responding positively to your faults and mistakes says far more about the strength of your character and ability to maintain composure under pressure. Every one will make a mistake at some point. But the person who can gracefully own, repair, and learn from their mistakes may actually find themselves with more opportunities than those who are childishly sore losers.