A wedding cake disaster in Tampa last year taught me to always pack a backup bag of fondant.
It was 95 degrees outside, and the humidity was a killer. I was setting up a three-tier cake at the venue when I saw the fondant on the top tier start to sweat and tear. I had exactly what I needed in my kit to fix a small tear, but this was a full-on meltdown. I had to strip the whole top tier, re-crumb coat it, and re-cover it with the spare half-pound of fondant I keep in a cooler bag. Took an hour, but the cake was saved. What's the one thing in your emergency kit you've actually had to use?