An almost disaster:
I did an event a few years ago at a private home in New Jersey. The groom was from Spain, and had hired a Spanish guitarist to play for the cocktail hour. Upon arrival, I noticed the guitarist did not have his guitar. When I asked him about it, he gasped, and said he had left it on the train. It makes me laugh out loud to think of how ridiculous this sounds now, but at the time, it was not so funny.
One of the most important things as a planner is to always keep your cool, and never to alarm the client. While I sent out one of our planners to get him a new guitar, I hooked up my phone with a Spotify mix of Spanish guitar so we would have music upon guest arrival. The replacement guitar arrived about mid-way through cocktails, and we gently faded out the mix for a seamless transition to the live music.