Well I think as a beginner there could be lot of mistakes which bakers can do when trying their hands on some recipe. But I consider this as a good because we always learn from our experience. So in my opinion following are the biggest mistakes which bakers do.
Butter and sugar should be well combined. One should stop at right time to avoid over mixing.
All ingredients should be at right temperature (mostly all should be at room but sometimes temp may differ)
Always use salt in the batter so as to take it to the the right taste. As chocolate if friend of salt so they go really well. together.
All cookies should be of same size and texture. Otherwise you will have different sized cookies.
When asked to put dough in to chill please do that. Don’t be in hurry.
Don’t overcrowd the sheet otherwise we could have cookie cake. :P Maintain space between each cookie.
Hope these points help someone who gonna try it for the first time.