Please tell me about a time when something major didn't go according to plan at work.

Since this is a behavioral question, you should utilize the 'STAR' response methodology. Make sure to stay positive, don't blame anyone else for the problem, and don't take full credit for the solution. Demonstrate how you worked with others to resolve the situation. As with any behavioral question, discuss the results you attained and possibly the lessons learned, especially if the outcome wasn't optimum.

"During a recent software update project, the versions of the software we ordered were not correct. We reviewed the purchase order and determined that the software had been ordered incorrectly. Since the software seals had been broken, we couldn't return the product, nor was the manufacturer likely to agree to correct this because it was our error. Even though this involved additional expense, my manager and I agreed it would be quicker to order the correct versions of the software so we could proceed with the upgrade. We did this and were able to install the software without any major delay in the project. This taught us to carefully review the software versions we had and needed before ordering new products or performing any upgrades."