A Not practicing their nutrition plan enough before the big race day or competition. 

Consistently practicing the same fueling plan that you intend on sticking to on your big day will not only give you more confidence going in, but also allows you to work out any potential issues. 

Research also shows that the gut is trainable. Meaning you can actually train the gut to tolerate and be able to process more calories each hour, but it needs to be practiced and conditioned ahead of time.