Not entirely true. The correct answer for this question is:
a) that someone highlighted an issue, you took on feedback, the steps you took to correct your behaviour, how you monitored and maintained this new behaviour and and how this has combined with the rest of your awesome skills to make you much, much better.
-or-
b) you self evaluated a deficiency in your skills, the steps you took to correct your behaviour, how you monitored and maintained this new behaviour and and how this has combined with the rest of your awesome skills to make you much, much better.
Commonly used answers to avoid:
a) I am perfect and don't have any weaknesses!! [interviewer wraps up interview quickly, never contacts you ever again]
b) "I guess I [insert any type of weakness here, it doesn't matter how big or small]." interviewee ends answer here.
The important part of the answer is that a weakness in your skill set (as long as it isn't a major, major problem i.e. I steal stuff and stay back after work to take massive craps in the top drawer or my co-worker's desks) was noticed - usually by receiving feedback or noting an area of your work that could be improved - that you made a plan to bridge the gap of expectation and reality, set yourself reasonable goals, put quality checks in place, self-monitored/asked for feeback (or both) and how you changed and maintained your behaviour. Lastly you mention the positive impact (give specific quantitative details of improvement. If you have figures give figures e.g. Improved sales skills: Q1-$1000 -> Q2-$2500) that this has had on that particular area and how it combined with the rest of your skill to produce a better overall package.
Realistically, you use it as a stealth brag thread. You had a problem, you either noticed it yourself or took on feedback (it's a win, either way), you changed your behaviour, you can self-evaluate and monitor your own performance and improved yourself. You are now a better candidate that has also let their prospective employers know that they are interested in self-improvement, can take feedback and respond professionally to it and are willing to change behaviour for the benefit of the company.
That's how you answer that question.
