I think one of the biggest mistakes is following the hype but not the fundamental. Very often we see prices rising a few folds even though nothing concrete was delivered by the project. During the fomo period in December 2017 and January 2018, we saw how a "rebranding" exercise can create hype and result in unjustified price pump.

Next, I think many people do not follow project updates closely when they are heavily invested. For stocks investing, we are often fed with information as a shareholder. At the very least, we get informed of the annual financial results. For crypto projects, the onus is on each trader to keep track of every project by following the project social media. At good project can turn sour very quickly due to a lack or a scandal. So do keep a lookout. 

Thirdly, it's that many people do not take profit early enough. Many inexperienced traders watch a Crypto rises 30% in a day and expect it to keep running up. Some might even fomo and add more to their position. Experienced trader will instead take some profit and wait for a retrace.

Finally, many traders do not have a consistent strategy and even for those who have, they may not have the discipline to follow it through. That's my own mistake that i am very mindful of as well.