Eric Smith – We are Principled: 6th Edition:
“There are many reasons to practice the demo but one of the biggest benefit is catching missed requirements. I couldn't list the number of times one team member would demonstrate a feature only to realize they had left something out or made a simple mistake, usually before the person they were demonstrating to could even catch it. The simple act of walking through the feature slowly proved immensely beneficial.
[…] Development, QA and management are all represented, and then somebody will ask QA: "So how come YOU didn't catch this bug?" It's said with an accusatory emphasis on YOU because after all it's QA's job to catch bugs and if they aren't catching them, then what are they doing? The poor QA lead fumbles around, mentions they'll add a test for it so it won't happen next time, and development goes on doing the same thing they've always done.
[…] If a mistake was made it's because development made it, not because QA didn't see it.”
(Via Rich Rogers.)