Early in my career a fellow team member working with me at a fairly well known fortune 500 company was testing out a process where using Microsoft Forefront Identity Manager we would cleanup in-active accounts and shuffle things around to various systems auth systems. Since this service was used to sync our prod AD and test AD there was a "connector" into prod. From this single FIM instance you could hit dev, test and prod ADs. Sadly there had never been any rules put into place to prevent a push from test -> prod.
On the day that this co-worker was doing some testing he somehow managed to push a change that he though was going to our test AD but instead went to the prod AD. This change ended up wiping out quite a few prod AD accounts. As in totally deleting them. All of our systems, including the phone system (not sure why) were tied into AD. All of the sudden people on our floor were saying they couldn't login to anything or send e-mail. Soon we found out that the CEO of the company was feeling the same pain and on top of that, was not able to receive or make phone calls. My co-worker took a look at the process he was running and realized he had screwed up big time. He killed the process but not before about half of our production AD had been wiped out.
Like most backup systems, restoring our AD from a backup had not been tested in awhile. Between figuring that out, since it naturally didn't work as designed, and having to get the backups from our off-site backup company most of the company was unable to do anything for about 8-10 hours. This included remote sites, field techs, customer support agents, etc.
What sucked is that this co-worker was one of the top members of our team and had been handed this FIM environment that somebody no longer with the company had built. On top of that he was not provided any sort of formal training and was really learning on the job. They let him hang around for another week or so and then let him go.