It all starts out with a detailed plan. Then someone says, “Can we deliver by October?” A few features get cut, some of the estimates get revised downwards and everyone gets back to work. Then you get to the end of September and find that someone removed all the contingency and that in the rush to finish the requirements a heap of important stuff got missed.
You spend days, weeks, quite possibly months pushing to get the software developed and, a few months before the real end, a crunch point is reached. It’s missing what’s now realised to be critical functionality; it takes an hour to process something that should be instant; the data doesn’t look like you thought it would; it’s too late for any real benefit to be obtained.