I think the biggest mistake many make is to buy a fish without studying them first (in theory and the actual specimen) and without considering if it is really suitable for their tank and their other fish. Many posts here kind of show that.
this is painfully true and unless a person is totally computer illiterate, there is no reason it has to be. we have so many sources available to us and it's a shame many people don't take advantage of them. impatience in this hobby never works!