Being emotional. It's hard to control your emotions especially once prices go down and you see all your money go down the drain. The panic seller's attempts to cut losses has only hurt them more in the process and makes them end up losing even more. Unfortunately, FOMO takes over and this is what kills the newbie traders. Using your emotions will probaly lead you to lose everything..
This is the biggest mistake of a novice trader. I myself still succumb to emotions when selling tokens of a project and can't do anything with myself. For long-term trading, you need to stick to your chosen strategy.