* (2010) When you're asked to restore last Sunday's backup to the dev CMS, make sure you're actually on the dev instance, and not, say, on the live instance. That literally every editorial person in the company uses. The day before deadline. (I got to restore 36 two-hourly incremental backups in sequence by hand. We lost only a couple of hours' work. But we verified our backups work!!)
* (2005) Never trust a UPS manual. Ever. Particularly, when it says that the "bypass" switch works smoothly, rather than, e.g., glitching the power and taking down all 75+ Windows PCs in the computer room. (The Sun boxes were of course unaffected.) Recovering the Windows network took most of the morning; the NT admins were less than impressed. And I was a contractor too. Fortunately working under the direction of the in-house admin.
The important thing being, of course, to recover and learn from the experience :-)