Keeping their bitcoins in an exchange is something that I would not recommend to begin with, along with moving their bitcoin around too much and not realising how much they are being charged for the privilege of doing so.
Leaving your bitcoin is bad because the bitcoin is not under control of you and your wallet but under control by the owner of the exchange you are using so you are relying on the honesty of the owner of the exchange.
Mistake : simple those who are not patient and just sell of their btc when they think its crashing
That is panic selling, let them do it we can buy the bitcoins form them and profit from them.