Every wedding has something that goes wrong. And while I certainly hope your something is small and manageable, it's best to be proactive. To combat any potential last-minute problems, prepare (or ask your maid-of-honor to prepare) a day-of emergency kit. The following items are what I've accumulated into my own kit over the years . . . if you have any additional suggestions, be sure to post them in the comments below!
What You'll Need
Clear nail polish
Pony tail holder
Excedrin Migraine (*Go with the extra strong stuff to combat a headache straight away)
Chalk (*For quick stain disguise on your dress)
Extra earring backs
Mini sewing kit (*I've used this at almost every wedding I've ever been in . . . Do NOT cut off the hanger loops from the inside of your dress. Instead, before you put it on, use two small stitches to secure them down inside your dress. They won't pop out, and you can hang your dress again for storing.)
Straws (*Once the lipstick and dress are on, only use a straw . . . You don't want to spill!)
Static cling spray
Mints (*Pop one right before you head down the aisle, so you're minty fresh for your first smooch as a Mrs. Just make sure it's gone before your grand entrance.)
A fake diamond ring and wedding bands (*Yes, I've used these before as well . . . They're a great decoy on a ring bear's pillow)
A granola bar (*You have to eat!!)
Roll-up flats (*Save your feet! It's a long day in heels)
Also, if you're short on time, there are a plethora of options available for sale as well. I recommend the Bridetastic kit from J.Crew—it's got 101 items divided into four compartments: beauty, fashion, personal care, and bridal.
Good luck! And happy planning!