It’s okay to not like all aspects of your job. But do not use this as an opportunity to get on the soap box and rant about how much you disliked your last job. Instead, pick one thing that you did not like and extrapolate on it. Do not talk about something that is the core foundation of your job function. For instance, if you are an accountant, you can’t really say you hated the number crunching aspect of your job and wished things were more automated.
To answer this question, pick something about the way your work or organization was structured. For instance, you can say that your previous employer was extremely inefficient in the way work was assigned.
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.