This question is similar to the mistake question. So first, let's think about what lesson we want to learn. A common mistake is when people make assumptions. I'll give an example where a poor decision was made due to making a bad assumption. The answer will also include what was learned and how the poor decision was used as a tool to become a better worker.
This example is geared towards sales, but like I said earlier, make sure to clearly explain the situation, identify the decisions, explain why it was a bad decision, and most importantly what you learned by making a poor decision.