Large-scale IT projects are dangerous
A typical example would be replacing a core business application system. After many years of usage and maintenance people believe they master the domain and have proven their competence in developing the system further through many incremental IT-projects. This sense of confidence, however, is most often unjustified when the organization faces a major discontinuity.
The expertise and experience required as well as the methodology and tools needed to successfully deliver a large-scale IT project are very different from ordinary endeavours. Due to the size and complexity of the project (i.e. the number of people and parties involved, the budget and the duration) even minor deficiencies have the tendency to lead to significant problems which will translate into major delays and budget overruns - if not outright project failure.
The history of corporate large-scale IT projects is littered with disasters which could have been avoided if a more humble attitude had prevailed towards the challenges lying ahead.