I would say, trying to learn every single thing and thinking to become the best in all of those one day.
I was a simple fresher, simple because I was just a blank fresher with a decent logical sense and able to code in C language to a decent level and a bit of core java(not HTML, no javascript, no CSS etc because I hated UI/UX). However, I have started to learn java(java EE probably) for my project. I tried to learn everything that the brilliant architect of our project has designed. And I ended up learning everything in pieces. I have become so immersed in that that I ended up reading lot of time and trying stupid things and finally understood that I've take too much already. Take it one at a time. I became the goto for anyone even the seniors if they wanted to know of something. In the end, the project ended and all my knowledge was left with me with little use.
This time it was javascript. This time I was careful, I started doing little things. When I felt, I need something for doing it better, I started learning again. Slowly, the beast to learn everything was back and ended up reading almost everything on medium and even, the tc-39. I was initially a guy who was very much against UI/UX and at this stage I understood that it was just different and not a nightmare. Now, after almost 4 years being a professional developer, I understood that I need to understand how to solve a problem properly than learn a language.
However, it is important to apply language specific optimisations which could make your code faster. So, trying to learn too much is my problem.
I can say, j has a great point which I will be planning to do next time when I want to solve something but that next time is never coming though.
Cheers :)