One of the biggest mistakes beginner cooks make is that they don't read the whole recipe before they start cooking. Lots of recipes call for an ingredient (for example, bread crumbs) but later instruct you to use half at the beginning of the recipe and the other half at the end. If you haven't read the entire recipe, you might use up the ingredient without realizing you're supposed to save some for later. Many recipes have extra instructions you might not notice in the beginning, so give the whole thing a good read through before you turn on the stove!
