Based on the given information, the most likely ingredient that caused the cream pies to have a spoiled flavor is the butter from Refrigerator A. Both Dave and Juliet used butter from Refrigerator A, and their cream pies had a spoiled flavor, which suggests that the butter from Refrigerator A was not fresh. If the butter from Refrigerator A was not fresh, it could have caused the cream pies to have a spoiled flavor. Butter can go rancid if it is not stored properly or if it is past its expiration date, and this can result in a sour or unpleasant taste. Since both Dave and Juliet used butter from Refrigerator A, it is possible that the butter was not fresh, which could have caused their cream pies to have a spoiled flavor.