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I've done a few, but one that sticks in my mind was trying to turn off my computer. I had a console open at a command prompt (I run a linux desktop) and issued the shutdown command. Threw on my coat and went home.
Turns out that I was logged in to my webserver at the time and had shut it down instead of my desktop. IIRC I had to call the datacenter to get it turned back on again.
Dumber: It set up some VMs for remote broadcast overseas on a Friday (the event was slated to start Sunday afternoon our time). I got bored after lunch on Sunday, so decided to check if the VMs had spun up, found out the gateway that controlled the VMs had died a miserable death the night before, and nothing had imaged. Spent the rest of the day re-imaging and getting things working so the client could get at least half the day on the VMs. Why so dumb? Never, ever check your servers on a weekend when someone else has the support pager. It's their problem. Let them waste their weekend.
Never, ever check your servers on a weekend when someone else has the support pager.
~ Years ago: Used Fetch (FTP; Mac) for site management (in addition to command-line). Selected a handful of files in a /subdir, clicked delete, looked away for a bit. Looked back, horrified: Fetch had jumped up a level to /public_html and was rapidly deleting away. Thank goodness for tape backups. But even now, using years-newer versions of the OS and app, before I click Delete I pause, fingers poised to Quit.
~ Last month: Crafted a dynamic robots.txt. All bots retrieved A-OK. Yay! Didn't notice for a week or so that ALL engines were getting the 'plain' version: Disallow: / . Not fun finding Google's Webmaster Tools reporting atop the "URLs restricted by robots.txt": [mymainclientsite.com...]