These particular items do not take much space in your kit at all. And as many as you know, I'm a kit condensing master. The smaller/compact/less I can make it, the better. These items should live in your kit at all times.
Nail Polish Remover - Models come in with funky nail colors or are told to have nails done but they still show up with them undone. Nail polish remover will save the day (and make you look like a genius).
Nail Polish - Well, you've removed that chipped nail polish. Now what? You have to put something on them. Always carry nude/beige/pink nail polish in your kit. Colors I carry in my kit:
* OPI - Suzi & The Lifeguard
* OPI - Samoan Sand
* Maybelline - Pink Prism
Tide to Go Stain Remover Stick - It happens! People spill things, no one is perfect. You get that one person that just couldn't resist sipping on some Fruit Punch while wearing a white shirt. Great if working with kids.
Mini Sewing Kit - I have yet to use mine but I know many MUA's who have come to the rescue of a bride who didn't notice that one of her dress sequins was falling off.
Mini Shaving Kit - Includes: Shaving Cream, Disposable Razors & Travel Electric Shaver. Whether you're working with men or women, always carry a shaving kit. Men have missed a spot, women forget to shave their armpits (yes it's happened). The electric shaver that I carry is from Conair. I've use it mostly to even out sideburns or the hairline in the back on the neck.
Headache/Cramping/Allergy Medicine - For someone who is deathly allergic to cats and is pretty much allergic to everything in nature during allergy season, I always carry my allergy medicine. I've been stuck shooting in a studio who happens to own a cat or shoot on location right smack in the middle of April when all the pollen is happily floating around. I've also shot with models who get cramps/headaches in the middle of a shoot. As a woman I know those pains are NO JOKE! You'll be seen as a hero if you have those on hand.
Fruit Snacks/Granola Bars etc. - They tell you lunch is at 1pm but it's 2pm and obviously lunch is late. You HAVE to eat. Many of us forget to eat while working (I know because I'm very guilty of doing it myself). It's important to have something with you to hold you up...just in case.
Hair Spray, Hair Brush & Pony Tail Holders - Even when I'm not doing hair, I always carry these items. Why? Hairstylists cancel or models are told one thing and do another. Don't let bad hair ruin your shoot. Brush it, spray it and put the hair up in a bun. Viola! You can still have a great shoot (especially if it's beauty).
That's pretty much all the "just in case" items I carry in my kit. These items don't take much space in your kit and if you make the travel section at Target/Kmart/Walmart/Whatever your best friend, you can find most of those things in a mini version and for cheap.