I mean did you ever reluctantly adopt a less than ideal solution because management told you that "customer is always right"?
I'll go with mine first. A few years back, me and my team developed a website of quizzes. Users create quizzes, answer them, rate/tag/vote/share etc.
In the same project, the team researched the best practices for improving front-end performance and worked hard to implement them. But all those efforts were in vain when the customer injected their heavy Flash based ads into the pages, making them load real slow.
I know it's not an objective question and very likely to be closed, but I would like to hear from the members of the community if they ever had such experiences.