Quoting from Hamlet is auspicious and foreboding start to this thread.
The last royal screw-up I've been solely responsible for was in preparation for a DR exercise. I vMotioned some hosts to non-replicated storage before the last replication run between prod and DR. And by some hosts, I mean the AD servers. We recovered everything except for AD, which made authentication... interesting. The worst part was that one of my coworkers had confirmed that the VMs were on the right storage just before I moved them.
It set me back a bit, and made me realize I had become a bit overconfident. Like many others, I realized that I couldn't edit my own work, as it were. I ended up making peer review part of the change management process, and added peer validation to the change implementation process. As long as you're able to improve upon the mistake, you're growing professionally.