“Is this acceptable? Not really. Is it understandable? Absolutely.”

When you visit parts of the website of the Driver & Vehicle Licensing Agency in the U.K. outside of standard office hours, you will see this: It seems ridiculous, almost like the famous 500-mile email bug, but Dafydd Vaughan explains how it came to be. It won’t be a surprise that the story starts with an old computer system, built around a day/night cadence where people would visit the agency during the day, the information would be collected, and all the data processing would happen overnight: At the time, many of DVLA’s services – particularly those relating to driving licences were still backed by an old IBM mainframe from the 1980s – fondly known as Drivers-90 (or D90 for short). D90 was your typical mainframe – code written in COBOL using the ADABAS database package. Most data processing happened “offline” – through batch jobs which ran during an overnight window. The team upgrading it in 2013 decided to not attack too many things at once, and swap the innards but keep the…