Y2K and all that
I saw in NYE 2000 standing in a customer's IT department, to be on hand just in case things went titsup.
Another customer had their bespoke pension system completely redeveloped to make it Y2K compliant, as the old one had been cobbled together in house using COBOL and duct tape some time in the late '80's.
When the UAT was underway for the replacement system, which involved running the old & new systems in parallel through the month end payment cycle, it highlighted discrepancies between the old and new systems. The new one was paying out more pensions.
Why?
Because some pensions were being paid to the surviving spouses of people born in the 19th Century, and the old code had decided that they weren't due to be paid a pension yet as it was getting the date maths wrong, and had been for who knows how long... I wasn't on the apps team so I don't know the details.
What I do know is that some of the OS features that were made available to facilitate Y2K testing, such as virtual date zones, would come in bloody useful right now.