I have two different answers for this. Very commonly, people set out for a new solution as their current one is failing in some way. However, immediately they begin to try to recreate the exact same system they're looking to replace rather than imagine their new, ideal solution. That's definitely human nature as we gravitate towards what we already know and what's familiar.
The second mistake that happens commonly is not identifying and agreeing on all app scope requirements. When identifying the scope to build a solution, often management and the business owners only ask for A, B and C. It's helpful to have all stakeholders sign off on what is being asked before building. That will help when stakeholders come back and ask for D, E, F and G. This always happens.
