I guess that one of most common mistakes related with the process of keeping coins. There are traders, who store all their coins on exchange wallet, which is very risky and not wise. For the matter of security it'd be better to leave there only small amount, for trading aims. It’s also ok to use exchange that provides a cold storage (I know that cex (https://cex.io/) does, maybe some other exchanges has such feature too). The rest coins should be transferred to a hardware wallet, or a paper one, they are more secure.