When I first joined my current team, the team culture was that a story card's scope was non-negotiable. In accordance with the classic tradeoff triangle (scope-time-cost), that means they were constantly exceeding the story card estimates. I slowly distinguished between two types of ways of looking at a card, and I learned something in the process.
Fixed-scope: We understand the end-user's need, and we'll spend however much time it takes to meet that need. In this case, we value a well thought-out solution over feedback--so we should choose this only when end-user needs are well understood. The end-result of this process is a story card that is done-done, and the subject will likely not be revisited.
Fixed-cost: We understand the end-user's need, but the schedule is more important than functionality. We'll cut out features, robustness, or complexity so that we can deliver something that at least partially satisfies the end-user's need. As a by-product of this process, we'll write new story cards for everything that we temporarily cast aside. In this case, we value timely feedback from the timebox over completeness--so we should choose this option when we're unsure of the best solution or when doing research.