According to the interviews that I've done in the last 3 years of Front-End Developers, Database Developers, and DBAs, the biggest mistake made is not knowing what they don't know and the second biggest mistake is not taking the time to find out.

Of course, it's not only the world of Developers that suffer from such a problem. I've run into folks with a Phd in Mathematics that couldn't convert 14₁₆ to Base-10 without a calculator, electricians that think that connecting a ground wire to the shiny spot on a cast iron pipe makes a good ground, doctors that don't know that OTC Naproxen Sodium works just as well as prescription Naproxen Sodium, and dentists that don't know anything about the devastating galvanic effect of dissimilar metals in a wet environment.

I guess that all means that the real mistake is one of attitude.