We have all heard it before. The project s going to take X months and cost Y dollars and accomplish great things. In reality it ends up taking 2X to complete, costing 2-3 times more dollars and accomplish only half of what was promised!
This kind of performance is not expected and tolerated in most industries with few exceptions such building a nuclear power plant or an state of the art space/defense system! This phenomenon is also not only applicable to internal IT departments but also many outside IT service and solution providers as well.
What does explain this phenomena? Why such things do not occur as often in other industries? Is it the newness of solutions and their underlying technologies? Is it lack of practical experience in part of solution architects to understand and spec out the overall solution more effectively? Is it the internal wishful thinking and sense of optimism of participants that wish such results could be accomplished in a shorter, less expensive manner? Is it perhaps some sort of a collusion by IT providers in selling us a bag of goods that is kind of too good to be true at times or is it perhaps lack of proper skills and resources in getting the job done right at first place?
If you ask me, I would say it is usually a combination of all above with a different mix in every instance. Realizing such risks and dealing with them in a real proactive manner, early on in the process, has proven to do wonders to IT project success rates.