My favorite version of this, is when a new version of something is greenlit by the business, and a bunch of enthusiastic devs look at the old system and decide

My favorite version of this, is when a new version of something is greenlit by the business, and a bunch of enthusiastic devs look at the old system and decide to rewrite. Instead of starting with the existing data, and determining how it's going to fit in the new design, they just make a new design and figure, "we accounted for most business requirements, so the data should be able to fit...". Then they set a launch date with the business, and just need to do that data migration. What could go wrong?