My worst bug was a typo in a single line of html that removed 3DS protection from many millions of dollars of credit card payments
I was working for a webhosting company, and someone asked me to rush a change just before leaving. Instead of updating 1500 A records, I updated about 50k. Someone senior managed to turn off the cron though, so what I actually lost was the delta of changes between last backup and my SQL.
I was in the room for this though: https://www.theregister.com/2008/08/28/flexiscale_outage/
It's like a single individual managed to delete the monolithic cloud where everyone's files are stored.
Thinking about it further the term "cloud" is a good metaphor for storing files on someone else's computer because clouds just disappear.
Flexiscale at the time was a single region backed by a netapp. Each VM essentially had a thin-provisioned lun ( logical volume ), basically you copy on write the underlying OS image.
So when someone accidently deletes vol0, they take out a whopping 6TB of data, that takes a ~20TB to restore because you're rebuilding filesystems from safe mode ( thanks netapp support ). It's fairly monolithic in that sense.
I guess I was 23 at the time, but I'd written the v2 API, orchestrator & scheduling later. It was fairly naive, but filled the criteria of a cloud, i.e. elastic, on-demand, metered usage, despite using a SAN.