I've had my share of SQL messes but nothing critical (thankfully!). Probably the worst as far as effect goes was a while back in a low paying dev job. I was under immense pressure to fix some thumbnails for an e-commerce site (as in, if this isn't done in 10 minutes, get your coat and get out). The shop I worked for was getting pressure from the client as they'd put it off for weeks at that point.
So.. I write a quick script to resize the master images and re-generate around 2,000 thumbnails. Except... I copy/paste the source path to destination - and I mistype 200px width as 20. Now we have a whole site with long thin product images and no originals to recover from! As in the linked story, no backups were in place and all work was done on production. Lost a weeks wages over that, and had to manually re-add everything from a stack of CD's :)
Lesson learned? Don't let pressure force you into making bad decisions. I knew I really shouldn't be doing that but I was young & foolish.