I don't typically do Internet chain mail, but Brian Kelley mentioned me in his post on his two mistakes and I'd hate to try saying I haven't made any mistakes. Good to be humble!
Mistake #1 was very early in my SQL career, we were moving from SQL 6.5 to SQL 7.0, and that change also marked the transition from the lead developer managing it to it becoming my bag. We decided that once the upgrade was complete, we would backup the data and reformat/reinstall the OS to bring it current, then load SQL on top. At the time it was running on RAID-5, and being the new guy on the job, all my reading said that was not a good idea. We broke up the drives into a three drive RAID-5 set for data, 2 drives in RAID-1 for log. Better, right? Not hardly. Turned out we badly needed those IO's that were now isolated on the log drive. Luckily for me we had room to add more drives and it wasn't an expensive fix. I think that probably counts as more than one mistake!
Mistake #2 was related to replication, the expiration time was set for about 3 days, and I came in after a long weekend to find that after a network failure I had about 200 publications that had expired. Lots of snapshots required to fix things. Some lessons there about expiration period, monitoring, and more.
PS: Oddly, the spell checker in Live Writer wants to convert Corbett to Corvette, even LW knows Jack is a high speed kinda guy!