This is a little easier than thinking about a mistake. You should make sure to include why it was a good decision and the result should be obviously meaningful. I also want to use this as an example of answering multiple questions. I would use this same answer for either of the two questions above. It was a good decision, and I fond a solution to a problem. Preparing for every interview question in the world will be too much work. Think of key experiences and apply them to multiple answers. In the case where it is the same interviewer, then you might be forced to think of a new answer. So it might be good to think of couple of answers.
For this example, I was careful not to use too many technical jargons that might not be understood. I also explained the situation carefully and explained why the decision was a good one. Remember that some of these answers will not work for you. In this case, if you never worked at Microsoft and never experienced this, just follow the steps of stating the problem, explaining the choices, why you made the decision, and finally why it was a good decision.
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