In my opinion, even if the initiating and planning process was extensive and very detailed, there are always some details, risks or problems that were never apparent until you reach the executing phase. In theory things might fit and seem perfectly put together and during the planning phase it is all about theory. It comes down to past experiences in the same fields that makes problems less. However, I still believe that that no matter how much you try to avoid there will always be something that can come up and change things around, otherwise the concepts of change documents wouldn’t exist.