"Tell me about a time you went over budget."
If you go over budget, it is a bad sign. It can mean you are not organized, do not plan well, or are not good with finance. So when you think of an answer, make sure you justify it with a good reason. Here is an example of what I am talking about.
Short Answer
"During our marketing campaign, we realized how many customers we were gaining through the radio advertisements. Although we didn't have budget for more radio advertisement, I still made the decision to place our ad on three more stations. We increased sales by 25% for that month, but I ended up going over budget by 5%."
Long Answer
"There was a project that had a strict 4 month deadline. I didn't have enough employees to complete the task, so I had to hire temporary workers. I was given a budget to either hire three extra heads for four months or to hire four extra heads for three months. Because of the importance of the project and the strict deadline, I chose to hire 4 temporary workers for 3 months to give us a time cushion at the end. We had a couple of unexpected obstacles and it turned out that we needed the temporary workers for another two weeks. We completed the project on time and everything turned out well, but in this instance, I went over budget by 5%. Due to the importance of this project, I didn't want to risk slipping the date. Even though everyone was congratulating me on this accomplishment, I feel I could have done better if I calculated a cushion for unexpected obstacles."
This answer is admitting to going over budget, but the reason was good enough. The answer indicates that by saying how everyone was happy and making sure to say how important the project was. In the end you say you learned something. But you can change this answer a little. If you want to be more clear on the importance of the project, you can say a monetary value, like it was a two million dollar deal, and going over budget by $10,000 was acceptable by upper management.