Finding out requirements by digging through the hierarchy endlessly or how not to implement a product. A recent upgrade included a migration from one database (home grown) to a commercially available product. It is well known that migrations are always dangerous and can be quite time consuming. The issue was that an attempt was made to replicate the existing (and mainly unknown) database into the new one rather than asking what was important to know after the migration was complete. The testing turned into a series of cycles while the testers dug deeper and deeper into the existing database.
Lessons Learned: Make sure to take a 360 degree view of the expectations before going down the testing path.