A friend of mine published a William A. Ward quote about optimism :

William A. Ward Quote

As an IT specialist, this topic rang a bell in my mind, about another quote, which was written long ago by Frederick P. Brook, but still vivid in our daily lives :

Computer programming [...] creates with an exceedingly tractable medium. The programmer builds from pure thought-stuff: concepts and very flexible representations thereof. Because the medium is tractable, we expect few difficulties in implementation; hence our pervasive optimism. Because our ideas are faulty, we have bugs; hence our optimism is unjustified.

Frederick P. Brook -- The mythical Man-Month


As per F. Brook quote, we're still highly prone to outstanding optimism in projects' estimates. Technical risks are elusive, not well anticipated, and when activities start-up, bottom-up estimates do not take potential rework into account and dependencies are not clear enough to secure delivery dates... This is worse with teams which are made of lots of junior team members.

Although an experienced Project Manager is not able to delve into technical details, it's his responsibility to ask the right questions to unravel the hidden risks, and to secure costs and schedule.