In 2008, Airbus made the new A380 quieter than any previously-manufacturered plane in an effort to improve passenger experience. Unfortunately, Airbus didn't realize until after they shipped the aircraft that the quieter cabin resulted in more unpleasant sound for passengers, in the form of bathroom noise, talking and other audible sounds (e.g. coughing, sneezing, crying). The result was a worse experience for both passengers and pilots, and caused Airbus to recall some of the fleet and reengineer the A380 once more to add more sound back to the cabin.

More info in the following articles and posts:

• gizmodo.com/5106074/airbus-a380-interior-too-quiet-eliminates-precious-privacy
• cbsnews.com/storysynopsis.rbml?pageType=moneywatch&catid=33540478&feed_id=76&videofeed=43
• seattlepi.com/business/article/The-Insider-Airbus-A380-cabin-may-be-too-quiet-1294016.php
• wired.com/autopia/2008/12/a380-is-so-quie/