[resonious]

Hopefully there is a product team or actual internal users you can interface with to know the expected behavior. I'd expect to at least be able to write some integration-style tests that can ensure everything works from the user's perspective.

If you're so disconnected that you don't know the user's perspective or have no way to see the program in action, then ..best of luck!

[madeofpalk]

Defining big obvious functionality is usually easy to get from people. It's all the finer details that people might not know. You might have a feature that's been relatively untouched for a while, but people still use, so the devs have either left or forgotten from their past "drive-by development".