Bothering too much about ratings. When my rating fell I was afraid to take a contest next time because I was worried that it would fall. Back then I should have realised that it doesn’t matter much down the line.
Reading too much theory about a data structure or an algorithm, but not implementation or sometimes too little practice. So I knew the ins and outs, but not implementation issues or the variants of the problem.
Self-loathing - Two years ago, when I couldn’t understand a concept I used to think that I’m stupid and that I couldn’t understand. That was the time when red coders seemed like super-humans. Some red coders were high school students!!!
I wasn’t satisfied with my mathematical rigour. I used to rely on intuition a lot which resulted in me getting Wrong Answer unexpectedly. That is why I stress it in many of my answers.
Laziness and lack of focus. When I sat down to learn something, I used to keep a lot of tabs open. Why? It is like my to-do list. If I get stuck in something I used to open another topic. In the end, I used to end up learning nothing.
Thinking that algorithms is all you need to be a good software engineer. It is wrong. I had no idea of the vast world out there. Nope, Competitive programming is just a sport. Have fun!
Focusing on quantity (in the beginning), rather than quality. Solving too many easy problems doesn’t help. I still do that, except that I write solutions in a language that I had never used before in my life. Recently I submitted some solutions in Kotlin.
8.In the beginning, I was scared of recursion and didn’t try to understand the algorithm. Instead I was more concerned with increasing the number of problems solved in my profile. But I must thank competitive programming here. I write mutually recursive functions pretty easily nowadays.
Trying to jump the difficulty levels at a time instead of going step by step. Obviously I could neither solve it nor understand the solution presented in the editorial. I used to learn the prerequisites or read the editorial only to give up after a few hours. I (under|over)estimated myself most of the times.
Being scared of Dynamic programming too much than necessary :P
I used to get nervous in the beginning of contests. In practice contest in Amritapuri 2015, I got WA for a question asking me to reverse a string!