Ran some little dinky BGP program on a PC that accidently manipulated routing paths in the building (circa 1993). Suddenly hundreds of engineers and employees found their network connectivity hugely impaired, if not outright disabled. IT tore their hair out, looking for the cause for several hours, until I noticed the program was running. After I shut it down, network connectivity was magically restored. The IT guy (Tim) told me that he had been able to use wireshark to catch the routers in the act of mis-routing traffic, but couldn't figure out why. I kept my mouth shut. Two weeks later, I met Tim again, this time in the parking lot as he was carrying his stuff out of the building in a cardboard box after being fired. I'm sorry, Tim.

Rebuilt a banks statements (was a vendor helping with an upgrade) without purging the previous file - caused the wrong check images to go to customers... eg i am supposed to see my deposits, i saw say my neighbors.

but i got a couple 1000d people free credit monitoring for a year from my company...

I was working in 2 racks, with 2 different systems. One in production, the other was being commissioned. I went to the back of the racks, to turn off the ones being commissioned, but counted wrong (long aisle of racks) ans turned off the production rack. There where 12 servers, 10 Front end and middleware, and 2 backends with an Oracle 10… I was white as snow from the shock. I was escorted outside of the datacenter, and given water. The pals with me thought I would collapse.

Uploaded a file which was part of an exploit as a resource for a unit test for the mitigation. Months later, the file was added to the windows defender definitions that stopped everyone from downloading logs from their failed tests.

Accidentally worked on something in master that should have been in a dev branch in git. Then I pushed it to master. Luckily, it didn't go into production. A colleague caught my screw up, and helped me back it out.

Pulled the wrong drive out of a RAID.