Lies, damn lies, and PERT charts
I've had the misfortune to be on a critically understaffed and underbid project, with a project manager who felt that if he just juggled his PERT around, he could make magic happen. Of course, when the chart told him flat out that what he wanted wasn't possible, he "adjusted the truth" - broke dependencies between tasks to allow them to run in parallel, had 5 times the tasks running has he had staff, and of course, the classic "No, I don't like your estimate, so I'm cutting it in half. Now, I've scheduled a meeting on why you never hit your deadlines...."
Like Powerpoint and outsourcing to contractors, PERT is a tool. Used correctly, and if you don't lie to it, it won't lie to you.
However, most managers are far more willing to lie than to accept the cold hard truth.