It's using something for the first time on the day it matters.
An event is the worst possible moment to introduce four unfamiliar products to your face. New formulas behave differently on your skin, in your light, under your camera flash. If it's going to break down at hour six, you want to find that out on a Tuesday, not in the photos.
Foundation is the highest-risk thing to buy new for an event. Shade looks different in store light, oxidises over a few hours, and can flashback white in photos. If you want a new base for the occasion, buy it two weeks out and wear it for a full day first.
This is the big one. A new retinol, a strong acid, a “deep cleansing” treatment, or a first-time facial in the days before an event is how people end up with peeling or a breakout in the photos. Whatever your skin is used to, keep doing exactly that.
A dramatic eye you've practised twice will look worse than the everyday one you can do half-asleep. The event is not the time to learn a technique.
This is where event money is genuinely well spent: a setting spray, a primer, or a long-wear version of a product you already use and like. You're upgrading the durability of a look you know works, not gambling on a new one.
Over a long night you'll touch up the lips more than anything else. Something forgiving that you can put on in a dark room is worth more than a perfect shade that needs precision.
Cheap, tiny, and the single most useful thing in a bag at hour five. Powder on top of oil gets cakey; blot first, then powder, and the face still looks like skin in the late photos.
The short version: decide your look from what you already own, add one product that makes it last, and test everything at least once before the day. That's it. The people who look effortless on the night are the ones who did the rehearsal.