This is still the most common mistake among newbies. People buy their bitcoins on coinbase, circle, or any of those sites, and they almost never do any other research than that, they think they are set and it's safe to leave the coins on there. Those people will learn the hard way.
It is really unfortunate for someone that is just coming into bitcoin and start using onine wallet without making the necessary research to know what bitcoin or cyptocurrency is all about and how to protect them from hack/theft. In every steps we do, we should always make good research about where we want venture else we are bound to regret.