Just last week a I witnessed again the great power of early feedback.
While starting up a small project I had several meetings with the overall project manager and several stakeholders discussing things like planning and needed resources. The project manager insisted that the work should be done within a certain time limit and that I should tell him the resources needed to deliver the project on time.
I gave him a idealistic planning based on his time limit and stated that after the first sprint a more accurate plan would be available. I requested several people to join the team and things like on site location, hardware for testing etc. Most of the requests were not fulfilled.... In the mean time the assembled team worked hard to deliver the first sprint. As expected the team velocity was not high enough to meet the project manager's expectations. After the first sprint demo, he could not attend, I called him up and explained that based on the current facts and team velocity, we would take three times the time he had in mind. The man was shocked! 'Thats three times the costs!; Why does this take so long?; What can we do about it?'. The result of this phone call was that all bells were ringing!! A few days later a lot of things that were not possible before where now miraculously not a problem anymore...
That the team could not make the expected deadline became obvious after just a few days into the sprint. But talking to the stakeholders about it did not had much result. I think that is because the arguments were based on just 'feelings' and the problems where just a 'possibility'. When we confronted the stakeholders with the hard facts of team velocity there was no place for them to hide.
By having a short feedback cycle of two weeks and being completely transparent we were able to change the course of this project and get most of the requests fulfilled. There could be more elegant ways to get through to certain people, but I doubt if they are this much fun!!