At my current employer, as part of their flavour of Agile (no two ever the same!) we developers are required to conduct a walkthough of every story we finish. Present must be a tester and a BA. All very well in theory.
But it seemed to us that some walkthroughs were uncovering bugs and/or oversights early, while other stories were breezing through their walkthrough but then exploding once the testers got their mitts on them. What was going on?
It turned out that in many cases, the developer, having spent possibly days neck-deep in the code, had a level of understanding of the problem domain far in excess of the BA and tester he was demoing to. As a result, the developer (consciously or not) would exude a confidence in his solution that would almost intimidate the "spectators" into not objecting to any deviations from the story specification.
A walkthrough conducted in this vein is almost useless, which led me to the formulation of the following rule:
The value of a story walkthrough or other such demonstration is directly proportional to the experience level of the audience