The stupidest thing I can think of is this:
At a place I used to work we had 8 Unix servers with almost but not quite similar roles and configurations.
After updating the printer configuration file for one of those servers, it somehow seemed like a good idea (for a few seconds) to distribute the updated file to not only the one server it was meant for, but also to the 7 other servers it was not meant for.
Doing this with a one-line command at the time was real easy, and just about half-a second after pressing ENTER it dawned on me why it was a really dumb thing to do.
After going to the basement of another building and getting 7 back-up tapes, bringing them to the server room on another floor, loading the tapes in 7 different tape stations, and running a lengthy restore process on each of the servers to get back to the correct configuration, I really had wasted a lot of unnecessary time.
All that work to recover from a one-second mistake!
It was all done after hours (with no overtime pay), so practically no one else but myself was impacted by that act of thoughtlessness, but I didn’t get home until late that evening.
It really is a good idea to hesitate before you push that ENTER button when you have root in a multi-server environment! :o