Neglecting the recipe order
Two cups flour, three cups sugar, four cups cinnamon, five cups baking soda. This is obviously a horrible recipe to follow, but the order is crucial. If the recipe tells you to add something before something else, then you need to follow it word for word. Failing to do so might lead to an unpleasant texture.
Order might not matter for some dishes, but if you’re baking, stick to the dang recipe! Mixing ingredients for breads and pastries is about developing gluten the right way. If you mess it up, you could end up with a soupy mess or something as rock-hard.