I once worked on an in-house ERP system which had been developed over about 15 years by various developers. It was the engine of the entire company, everything passed through it. The CFO and some senior leadership erroneously blamed it for their shortcomings/used it as a scapegoat. When new management took charge, an initiative was started to replace the system with an industry standard solution. Both myself and the CTO (my boss) made it clear that we strongly felt this would not only go way over budget, but ultimately fail as a project.
Having no understanding as to the technicalities involved, the project was given the go ahead by the directors after several meetings with a vendor. After the CTO and I expressed our concerns about the scale of the project and the sheer amount of functionality involved, the vendor gleefully assured us that they were experienced with "migrations of this scale" and were more than prepared, which was music to the ears of the CFO.
Daily 2-3 hour meetings followed (for many months) to define the scope of the project. Within each meeting I sort of zoned out because it became very obvious that no only did the vendor not understand the scale of the work involved, but had started cutting corners everywhere/leaving out crucial functionality, and this was just the scoping stage, no development had even started yet.
I eventually departed the company but kept in contact with the CTO and learned that after 5 years (project was scoped for 2), the migration was abandoned costing multiple millions of dollars with nothing to show for it.