To err is human. So, it is perfectly OK if you committed a mistake at work but before answering the question analyse the magnitude of mistake you did and the effect it had on the company.
What is more important is what did you do to rectify the mistake and make sure that you dont do it again?
So, mention the mistake you committed and keep the focus of the answer on the steps you took to rectify it.
Try to put the ball back in interviewers court by asking him about the salary they offer for a position like this. Most of the big companies will have a fixed remuneration for each level.
However, if this is negotiable, you will have more negotiation power if you have some work experience. So, know your lower limit (amount below which you cant go) and also know the maximum salary in the industry for the position then put forward a figure which is not very exact. It is better to mention a range. For e.g. if you are expecting something around 55 K, say that you expect something in mid fifties. Dont keep the range to broad otherwise you will be offered something towards the lower end.
If you are a fresher, most of the times you will have to accept the companys offering for the position. However, if you find it too less, you can definitely discuss that during the interview.
Here you should discuss a stressful situation that you were able to overcome and keep a positive tone, do not say you never came across a stressful situation. Typical answers can be: