Lots of concealers. Concealer can be used for so many other things other then just concealing. It can make a great cream contour/highlight product and if you can't get your foundation to match your client then mixing in some concealer can help create the perfect colour. It can also be used as a eyeshadow base (depending on the concealer).
I always always always carry around OCC lip Tars in the primary colours (red,yellow,blue) and black and white. That way I can always mix the correct lip colour if I don't have the right one on me.
Mascara wands, Cotton buds/pads, make up remover and tissue. These are a given but you would be surprised the amount of time I have forgotten tissues.....
With that being said plasters are great to have, they can save a models poor feet and make you the best MUA in the world to her. Also painkillers, photography lighting can be a nightmare for bringing on headaches.
Cream blushes, I always have a handful of Illamasqua's on me, these are great as a substitute for lipstick and if you want that matchy lip and blush combo.
Glycerin, a few drops of this mixed with some pigment makes for a lovely paste like pigment which can shake up a look. And don't forget you can mix pigments too.