Not just research on the animal they buy, but the breeders too. I've seen too many people who do everything else right pick up an animal from a bad dealer or petstore and everything goes downhill because they started off with a sick, wild caught, or unsexed animal (thinking mainly about tarantulas with that last one).

I don't even bother with researching caresheets anymore; I go straight to the most active forums for a species and describe my setup and ask for everyone to nitpick it before I get an animal.