Can you give me an example of a campaign that did not work out as you had planned?

Use examples that reveal your ability to adapt, detailing how quickly you reacted and the logic behind your course of action. Avoid examples that reflect poorly on your performance. Describe the situation, the obstacle, and how you ultimately overcame it.

"We'd done some pro-bono work for a local non-profit. We were interested in converting some of their donors into clients for our firm. There was a thank you function after the fundraiser. We'd provided a bit of swag for this event and were looking forward to networking with the donors. After everybody arrived and the mixer was underway, the guests got alerts on their phones to seek cover due to a tornado warning. The weather turned ominous, and the benefit didn't go as planned. Later, we converted some of them to clients, but not to the degree we'd hoped."

Providing examples where you dealt with disappointment is fine. How you deal with disappointment could be the point of your answer. But this is also an opportunity to show yourself as a half-glass-full person, by sharing an example of a campaign that worked out better than you had planned.