I worked on a website where my goal was to increase conversions in an registration process. There was an input field that I wanted to alter "Date Picker" as my analytics and observations showed this did not provide a good experience
The date picker input field was used for entering your date of birth. The problem in Canada is we say our birthday and possibly think about our birthday in order of month/day/year. However the interaction the user had to make with the date picker was year/month/day. My observations was that users couldn't figure out how to change the year let alone efficiently (switch to decade view). This ultimately lead to slow registrations, incorrect birthdays (which was crucial to be correct), and users giving up.
The Design Debt
This improvement recommendation sat in our backlog for over a year. I had a hard time keeping mockups updated as mine showed the new date of birth input field but whenever we created mockups for client sign off, we had to show the way it would look as upon delivery not as the way UX wanted. This often leads us to maintaining two mockups at any given time.
When the input field was finally implemented we observed a 1.2% increase in conversions. We were able to retroactively analyze how many users we would have gained had this UI pattern been implemented a year ago. We then took this number and subtracted the development/design effort to make it a reality. moving forward the UX team is now given more klout when it comes to UX/UI recommendations.
Although this is one example, there were and are many UI patterns that the UX team has requested to be updated as part of various changes to the product however only some are implemented. We spend a lot of time maintaining the UI mockups as we would like to see it (design debt) and mockups that are what we will actually deliver to client.