First and the most common is that they will not do their due diligence or do they own research. Because that is also how I hustle when I was new to trading and every altcoin projects reach their moon so I thought to myself that since it can happen to any coin then I will just listen to the most people who shilled the same coins and will buy it. Not gonna lie that it worked but everything now has changed. You need research and please do not put your money on projects that has only beautiful whitepaper.
Yes, I agree with what you said one of the common mistakes and problems of traders/investors is lack of own research. The other common mistake is emotional problems ( rushing to buy and sell without a complete analysis ), risking plenty of money ( we know that in trading you should prepare to the part that you will lose your money and you should spend only the money that you can afford to lose) and also don't assume that you will profit easily especially in crypto trading. 
Most of the people who are in trading will always aim for high returns without any research on what they are investing. In the beginning, I used to think in the same way, but after somehow I realized to do some research on what I am investing. Without a specific idea about the project, it is impossible to make a profit in the crypto market.