There is no danger in admitting to making a mistake. However to correctly answer this type of query there are certain guidelines that you should follow. Firstly always mention that you identified where you went wrong and have learnt from the experience. Secondly try to give a example of something that happened as far back in your past as possible. This way you can shrug it off as a youthful error and something you would not fall for now that you are more mature and experienced.