Adding just one more can save your budget.
Every event planner packs what they need for their event, but some forget to bring a backup for essential equipment. This is especially important when planning events and conferences outside of the U.S. because things are not always as they are in the States. This is usually a lesson that most planners (and attendees) have to learn the hard way (me included).
I have just returned from an incentive program in Turks and Caicos, where one of our attendees forgot to pack an extra MicroSD card and got quite a (sticker) shock. It was so bad that I had to take a picture.
If he were back at home, this would not have been a big deal, but because we were in TCI, there was a pound of flesh to be paid for not packing just one more (and yes, Turks and Caicos are on the U.S. Dollar).
Price for 4GB MicroSD Card Amazon:

Price for 4GB MicroSD Card in Turks and Caicos

Wow! That just sucks; no other way to describe it. At least they had a sense of humor.