Advertised an OS deployment to the 'All Wokstations' collection by mistake. I only realized after 30 minutes when peoples workstations started rebooting. Worked right through the night recovering and restoring about 200 machines.
Plugged a server in after it had been repaired but the person whose responsibility it was insisted it would be fine - they didn't release the FSMO roles from it, the time was an hour out, it changed the time EVERYWHERE and broke ALL THE THINGS. Not technically my fault, but i should have pushed harder for them to have demoted it before I turned it back on.