It is a mentality thing. Fear of missing out and fear of losing all. Especially for beginners, I've been there before. I used to have zero trading plans or just can't stick to it once the market was shaking up but it didn't break the pattern that I initially observed.
When they see some coins to increase in value then they think that they will miss this chance and then start to invest without thinking about the price that how higher it is. On the other hand when they hold any coin and price reduces they sell their coins with loss due to fear of losing more so in both cases they accept fear but don't utilize their strong mentality.
Newbies are more likely to do this mistake because they don't know everything better as an experts knows and their main aim is to earn in a second which is not possible so in such greed they loss their money without realizing that its not the fault of crypto but its their own fault.
Buy High Sell Low
This is usually the things that newbies would really be able to experience due to lack of experience and knowledge on things on where they would be making those buy decisions
when they are already seeing a long green candle and this is where they would really be having that kind of feeling or emotion that they might be able to miss out something and this is
really that something that could really happen on this case. Buy high sell low, you would be finding yourself that being a holder once you do see those negatives.
The worst is that you do cut loss just because you are assuming that the price would be going to zero, but well these are the common newbie mistakes but sooner or later
you would really be able to realize those things and changing up your insights right away.