This may be unavoidable at times because people get upset when they're proven wrong, and they shut down and get defensive.

For example, a woman was unhappy with some work I did. This was not my fault, but a result of her poor planning and inherent challenges with the venue. (Event photography) I send her a very polite email explaining the circumstances and challenges. This scenario raised this question in my head.

How can I disagree, and be right without also being an asshole? Is that even possible? Do any of you have experience with this dilemma?