Once upon a time, there was an Agile team that was working on stories in a sprint. The team was going on fine, until the product owner decided it was time for him to go on a month long vacation (and then leave the company).
Life Goes On
So, being a good scrum master, the SM appoints a new PO to make decisions on the specifics of the stories that were already in progress (and some finished). The new PO was a pretty sharp fellow, but he didn't have any background for the feature, so he was a might confused. But, the team had fairly good direction on the feature, so they persevered and completed the tasks as they understood them.
The Shadows
Now then, from lurking in the dark shadows, there emerged the original PO's Boss. The Boss of course looks at the feature nearly complete, and decides that there were a couple minor tweaks here or there - hey, he's the boss - he can do that. No worries, the changes were slight, and easy. Done deal. All the stories were completed, as well as the automated acceptance tests that verified the now-completed tweaks, as well as the remaining story criteria. All the words had been blessed by the Documentation folk, the UI had been blessed by the User Experience folk (this was the Second iteration of the UI design also BTW).
Demo Day
At the end of the sprint, we all get together with the interested stakeholders in a room, and review the stories, the functionality that was delivered. The managers, the development director, the project team, scrum master, and QA were all represented. Keep in mind now, that everyone has actually seen the feature at least once... "We can't have it do that..." says the boss. Well, apparently a modal dialog with OK and Cancel doesn't work the same in Boss-World as it does everywhere else. So, he fires up his argumentation engine and proceeds to corner the entire meeting with a redesign of not only the UI but also the functionality of a standard modal dialog. Nothing was up to par for the Boss, and - remember - he had seen it all demonstrated for him before...
The Outcome
None of the stories in the sprint were accepted. Sprint velocity: ZERO points. Bad day. Alcohol was required.
The Moral
The moral of this story is this: Have a Product Owner who knows what the feature does. Make sure the product owner has input from ALL the stakeholders - oh yes - in a TIMELY manner as well. Make sure your scrum master has the ability to keep lurking skeletons at bay. They can have their say in the next sprint. But at the end of the planning meeting, the stories should be pretty much fixed and everyone should understand the acceptance criteria for them. Stories shouldn't just have arbitrary criteria appended, grafted, attached, pasted, or otherwise affixed to them after the planning meeting, even by skulking lurkers.