"Tell me about a time you aimed too low."
I have never been in sales, but the example answer I want to give is an example of answering the question while displaying good traits. Remember to be clear when explaining your experience and don't assume that they will understand everything. You should explain it step by step.
Short Answer
"There was a new project that I was interested in. Because I had other responsibilities, I didn't volunteer. I wasn't too busy, but I was worried I wouldn't complete the assignment on time. I realized I missed a wonderful opportunity because I aimed too low."
Long Answer
"When I was selling cell phones for Verizon Wireless, I had a great month where I reached the quota in just two weeks. My goal is to always break quota by more than 10 percent, and this month, I new it was going to be easy. I scheduled to take a week off to relax, and at the end of the month I surpassed the quota by 31%. I had an opportunity to break the location's record, but I didn't jump on the opportunity. I was basically thinking that having achieved my original goal of 10% was sufficient. But I think when opportunities arise, I should be flexible and willing to change my goal. I aimed too low and I missed a rare opportunity."
This is not a bad mistake. You can say you were burned out, or needed to take some time off, or wanted to rejuvenate yourself. This answer doesn't show a failure, but shows a great success. In this example, aiming too low resulted in great numbers and great results at the end of the month, so there is no harm done. In the end you are indicating that you could have done even better, that you are willing to re-evaluate your goals, and that you are now able to see and jump on opportunities.