When I started gathering requirements from the city's rep who was my point of contact I asked them for any documentation they had such as use cases, functional specs etc. All they could provide was the original COBOL source code bound up in those huge 17"x14" binders; the one's that have the holes along each side from the old dot matrix printers with every other line having that faint light green background.
Well I'd never seen COBOL in my entire live and I could only look at the code for about an hour before my eyes started to bleed. Fortunately I was able to get restricted access to the system and it ended up being pretty easy to just reverse engineer the thing by mapping each green screen of the app to a corresponding web page. In the end I didn't need any original code or documentation to do the job.