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I would say that the most common mistake I have seen in IT projects is bad management of expectations from the various stakeholders. And I mean the outspoken as well the hidden ones. Expectations from the business side and from the IT side. When these expectations are not well managed I have seen solutions delivered that are not accepted though they from a technical point of view are according specifications. By arranging workshops and other ways of communication with the stakeholders and by listening twice as much as speaking a new IT PM can start getting it right.