If they are a day trader then yes but for the other type of traders mistakes can rarely occur on them most especially if they mastered it already. It's fine to make mistakes as long as you can correct them as soon as possible and make more than what you have lost due to your mistakes.
Traders who are hurrying can be a newbie trader because experienced traders are organized and will execute things more calmly. They know that rushing cant do good on them. I think you define FUD wrongly. It must be FOMO where people rush and buy at the top but FUD can cause for someone to sell at lows because they are too scared and thinks the price will dump more according to what they heard.
It all depends on the situation, maybe sometimes you need to sell on a fall in order to save at least part of your funds, this may apply to force majeure cases, like the fall of Luna, but this is really an exception to the rule. A trader who trades for a long time and receives a profit from this can be called an experienced trader, which means he follows the rules, and knows how to do it. If for a long time the trader has only losses, then this is just a gambling and it will not last long.
But how you would really be able to determine if a certain coin would fall just like on what happened to Luna? No one could able to determine that which people would simply hold because we do believe that zero
value isnt really that commonly happen into this market which simply means that people would really love to hold because there are really chances for us to recover which is a common thing.
Errors made or mistakes are that common but we could reduce out the odds if we are really that able to learn up things and avoid as much as we could.