I once gave thousands of government employees in Norway a long lunch break from what should have been a non-locking migration. Or so I thought. Apparently it wa

I once gave thousands of government employees in Norway a long lunch break from what should have been a non-locking migration. Or so I thought. Apparently it was fixed in 12b (or something, a few years ago), and my tests in staging went fine. But in production we didn't run 12b as in staging, but 12a... Can't remember all the details. But something about adding a new nullable column with or without a default value of some kind. It had to lock the table, but multiple queries were already running. Those queries had some locks already, but then needed those rows the migration had already gotten hold of. Leading to this huge deadlock I had no idea how to solve. Worst part is, it was one of the first times we tried deploying during working hours. At the time we normally only got to deploy 4 times a year, but we pushed for a more modern approach. Luckily the server guys were on our side, and a few years later that government agency is one of the best technical places I've seen after a complete revamp.