Never, never race with new gear or equipment that you haven't tested in training. Never use combinations of gear or equipment that you've never tried in training.

I used a pair of snowshoes with my cross country ski boots, which I had never tested in training. This caused us over an hour of lost time in a 4 to 6 hour race, from the snowshoes falling off of the ski boots.

Same goes for food - always test certain foods in streneous training before using it in a race. You need to know how your body will react to certain foods under extreme physical exertion.