Mistake : simple those who are not patient and just sell of their btc when they think its crashing
This is the biggest mistake and time will teach them how to hold bitcoin and being rich with bitcoin. I think the second biggest mistake is that they do not very care about the private key. Private key is very important to bitcoin holder
I think everyone experience this one, when we all new here we are really so afraid and let our emotion to control us but this is a good lesson to learned. Sometimes we need to experience this early in your career for you to become stronger the next time you encounter things like this.