Collisions between Agile and real life

1. Some collisions between Agile ideas and real life business that come to my mind:
   The importance of oficial "sign-off" before moving to the next step is commonly recognized. Otherwise, you can't control the process: there is a great risk of complains that team is doing wrong job. You will end up arguing about things you took for granted when it's too late to change them without wasting a lot of time and money. But this tesis conflicts with agile practices of overlapped stages/phases and iterations when you move to next development stage before previous stage fully closed. Without overlapping stages we go to "waterfall" approach!
   
   May be the solution for the problem is that upfront sign-off/approval required only for key documents required before actual development can be started: business plan, vision statement, overall project plan.
   
   Additional thought here that overlapping stages approach also conflicts with requirement (from "agile" iterative incremental development mothodology) to continiosly have a releasable product with good quality. It means that we have to stabilize the codebase each time after change is introduced before going to next change. This is why "entry and exit criteria" offen used for milestones/legs/stages.
   Oops, seems it conflicts timeboxing principle? I think timeboxing in this case means that we have to revise "exit criteria", understand what was left-out and make a decition according our current "best understanding" should it be done ASAP in the next timebox or postponed. So in this case "exit criteria"just allows us to have good understanding of differences between original expectation and realization.
2. Here is a story I head from my friend just yestaday how timeboxing principle was violated. My friend is working for sw dev company who adopted SCRUM practices thru whole enterprise. But management forced to extend last sprint when they realised development can't compete all features that where estimated for this spring. This story is the key to question why "waterfall" is still so popular. Business what to have good prediction and plans!