For beginners, every mistake costs them an entire stack. Sometimes, you don't tend to take the magnitude into account. So, even a small mistake which may not come into your consideration of mistakes might cost you a small pot which is again a part of your large stack. Mostly, people don't tend to correct these mistakes which leads to an aggravation of that mistake again. These kinds of mistakes don't happen often but cost you a small pot every time which get expensive in the long run.