I don't know what monetary cost they assigned to this, but this is the one I got in the most trouble for.
Frankly, it was something I got blamed for. I guess I can take partial responsibility. You guys tell me.
I was the only UNIX guy at this place. We were moving our Main Internal Server to a newer machine. I had set up a cron job to rsync all user data nightly, so that when we transition over the rsync would be faster.
So, the big day comes. I come in on a weekend, do the final rsync, change some DNS entries, shut down old machine, bring new machine up. No problem.
Next day everyone is working happily, everything is working smoothly, no worries.
Or so I thought. Turns out the main developer wanted something off the old server, so he turned it back on to copy his files... and then left it up.
So, during the night, the thing automatically rsyncs and overwrites an entire day's work for about 80 people.
Definitely partially my fault for not disabling the cron job, but I was the only one who got in any kind of trouble at all for this (to the extent of almost losing my job, and frankly that was the catalyst for me leaving that place).