Just found a humongous bug in production. The customer relation number, which should have been unique was shared by ten customers.
So instead of 400,000 relation numbers we only had 40,000 unique numbers.

The system is live for 3 weeks now, and we just fixed it on a friday at 5:30PM

If we had found out a week later every customer would have gotten a plastic card with the wrong number, because the cards will be printed in four days...