Gall's law :
"A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system." - 1975
This is one point which is why, when I see a over exited young project manager declaring he will develop a remarkable system, I know he will fail.
This is one reason why I don't follow the conventional methods. Rather go for the simplest possible solution.