So it's the day before your big race and you're at the expo. You spot some really cool shoes you've been wanting to try out. You buy them and throw them on your feet the next day for your race. WAIT!
Rewind that scenario--go to the expo, enjoy it, maybe even buy a few things. But don't try any of them on race day.
When you're racing, there's a hard and fast rule--no new is good new. You want to make sure that everything you're wearing, ingesting, doing, is something you have tried before. That means no new gels. No new shoes. No new super cute skirt (sorry fashionistas) unless it's a model you've worn before.
Race day brings with it stress you wouldn't normally have on a training day. If you add something into the mix that isn't tried and true, there's a good chance it could come back to haunt you. The new nutritional products could cause GI distress. The new clothing might cause uncomfortable chafing. Running a different pace than what you had planned and practice could result in a blow-up.
After all your hard work to get to the start line, you don't want to see your race fall apart because you decided to try something new. Stick with the tried and true and then a few days after the race, break out that new shirt or whatever fun thing it is you bought at the expo!