Many new people to trading often want to make fast money with small capital trying to get rich overnight and that is the first biggest mistake not understanding how the market works.
2nd mistake often related to account funds,margin and traded lots size were they tend to increase lots size wrong in relationship to account funds and suddenly gets a margin called.
3rd mistake,they don't seems interested to learn and get familiar with basic important terms such as spread,slippage,account stop levels. What is pip,point,ticksize,news...and how all this impact an order placed.
Long-term investments in my opinion is the best way to grow a capital along with good money management and strategy,steady and safe. Patience is a key factor which many don't have but as usual in many cases - driven by greed they dig their own hole to fall down in and eventually loose it all in lack of knowledge and understanding. That then becomes the 1st hard key lesson to learn and keep in mind and fall back to.