That's a great example. There are a lot of implicit requirements that result from business processes, hardware configuration, org structure, and so on. Nobody really even thinks of these things as requirements at the time, and reverse engineering all that later on becomes impossible. This a great article on this phenomenon that likens long running software projects to living on a generation ship :)

https://medium.com/@wm/the-generation-ship-model-of-software-development-5ef89a74854b