Quitting jobs and moving without having the next job lined up. I always figured I was a smart guy, I could land on my feet wherever I ended up. And that was true, but it took some time and effort to get a job that lined up with my career goals, and while that was happening I was pretty stressed out. I also feel like I've lost about a year of growth (and salary!), which is hard to let go of.

I realize that this isn't really a big revelation, but it didn't really hit home for me until I got into a real "career" -- it wasn't such a big deal when I worked retail jobs.

discovering hackernews?

but seriously, probably thinking i had to "pay my dues" before I begun my career. Thinking I wasn't good enough to get a job, and I had to spend time at Uni first. It wasn't until I was mid way through my masters before I realised that a job wasn't going to magically appear the day I graduated. I dropped out and never looked back.

Also, thinking I had to shoehorn myself into the industry by working my way up to it from tech support. That was fruitless.

Not buying that Perl book at the college bookstore in 1994 and not really digging in deep when I first encountered RDBMs at about the same time. At the time I was learning grad level social science with a quantitative focus - lots of statistics with SPSS. Around 2012 I started adding in HTML, PHP, Javascript, SQL, R and now working on Python. I was right on the verge of putting together the multi-disciplinary skill set that serves me well today as early as the mid 1990s.

Not taking Statistics in high school. Ended up taking it in College, but found I enjoy it quite a bit and would have been able to enjoy it nearly a decade earlier.

Not learning web coding earlier. Although it wasn't a conscious choice, just something I wasn't exposed to.

Taking such a long time to get my degrees. When I finally had the educational background I wanted, I found it's difficult to get those entry-level positions when you're over 35. And if you can't get those entry-level positions, it's extremely difficult to have a career in the field you want.

That time I had to call a lot of SOAP APIs. Not Java SOAP, but .NET SOAP. There's a difference and I regret having had to know what it was.

Accepting the first offer that came through, every time I went job-hunting.

Nearly drowned in toxic work culture..

work for more than 10 years on the same company and almost with the same technology. bad mistake.