1. Not knowing the exact number of calories, (or fairly close to exact), that you're consuming in training and on race day. Without actual quantification of what you're taking in, it's nearly impossible to know if it's too much, too little, or just right.
2. Not keeping a training log which details the nutrition you used and tried, so you can perfect it.
3. Testing your nutrition plan for race day when training at an intensity which doesn't match race intensity. On race day, if you're going harder than you are in training, then don't be surprised if suddenly your stomach doesn't jive with what you're putting into it.
4. Using a concentration of calories which doesn't match what you'll use on race day. When the concentration changes, the interaction of the calories with your stomach changes, especially if the concentration is stronger.
5. Many top athletes follow a strict diet, only to change it entirely during the week or two before the race, claiming "carbo loading". Worst thing you can do is dramatically change your diet before the race. Don't do it. It's worked fine for you, stick with it.
6. More is not better. Less is better. Try to get your body trained to need less calories during a race.