Trading too much and too impatiently is the number one mistake. Trading with no idea of technical analysis. Most people lose money by trading. When you trade you have to beat the market. Very few do and those people are bound to be much more skilled than much luckier than you. So, don't.
FOMO buying is probably the worst. You end up buying on a spicke then your entire investment looses all value.
Also investing in a coin that they did not reserch and it turns out to be a scam.