Mistake #1: You don't pre-treat your hair.
Maintaining healthy, hydrated hair starts way before you turn on your flat iron. Always pre-treat wet hair with a thermal protectant spray as a shield against the heat. If not you could be setting yourself up for some major damage later.
Mistake #2: You select the highest possible heat setting.
Just because it goes up to 450 degrees does not mean you should use that setting. For smooth and healthy hair, stay in the 300-degree range whenever possible.