This same exact story of a UI disaster happened at a Fortune 100 company I was at in 2005. They had a legacy app (think DOS character mode talking to a mainframe) for entering accounting documents.
To replace it, a high-priced consulting firm installed a "web portal" technology. (Unifying a company's various enterprise applications behind a single web portal was a hot endeavor back then.)
When it was rolled out, the accounting department fell behind on their work. Nobody noticed that the constant UI banner at the top of the of the web portal browser window was forcing users to always use the mouse to scroll up and down to look at information. In the old DOS system, they could just enter info from invoices quickly without even looking at the screen. But the new system broke the users' fluid familiarity.
The UI disaster was so bad that the consulting company had to have their consultants come in on Saturday to help the client's workers catch up on all their data entry. Imagine an army of $150k consultants doing the work of $10/hour clerks because the web portal's UI was never really field-tested with a real-world workload! If just one UI web developer actually shadowed one of their users for a day, they would have noticed the usability problem.