Worst Mistake: I let the stress of my former job get to me so much that I became a completely ineffective manager and took my anger out on my employees. When this was finally pointed out to me, I voluntarily stepped down and changed jobs.
Result: Not only did I pretty much start my career track over from scratch by changing positions, I treated some really good people really badly in the days leading up to it.
The Bright Side: I love my new job, and it's given me a chance to learn a lot about things I never would've learned about otherwise. And I've met some very cool people, and have been able to form friendships I never could've in the position I was in before (that whole "fraternizing" rule...). And, to top it all off, now that I'm getting good at my new job, I'm actually making more money than I ever could've in the position I had before...