Mistakes happen and today one has just happened to you. It is a serious problem and you have to inform your superior. Such a situation is not pleasant but you can handle it without damaging your future career with the company. Keep in mind that the mistake should very quickly be put right and not kept secret for as long as possible. So take a deep breath and be inspired by the following recommendations.
Right time and place
Yes, it is necessary to act immediately. But this does not mean you should tell your boss the bad news just when (s)he is coming out of the elevator or negotiating with clients. Schedule an appointment for a specific time and do not forget to stress it is urgent: this will show you take the whole situation seriously.
Thorough preparation
Your boss will want to know why the mistake happened. You should, therefore, prepare information on where exactly an error occurred or at least on how to ascertain this as quickly as possible.
Clearly and concisely
Briefly describe the problem and its scope. Be very precise and do not try to hide what would come out anyway. Do not blame anyone and avoid strong emotions.
Possible solutions
Do not think your task is over once you have informed your superior of the problem; on the contrary, show that you have already taken steps to remedy the situation and describe what further measures you propose. Be as specific as possible regarding how long each step will take and what the measures will bring.
Brief apology
Too much apologising may harm you. Apologise briefly, then focus on dealing with the situation and other positive things. Just eating humble pie will not solve anything.
-kk-