Jamielynn: A gallon sized Ziploc bag for blowout outfits after you rinse them out. This has saved me from trashing so many nice outfits!
Courtney: Chopsticks. They mix formula great when you're out of the house all day, without making it too foamy!
Lindsey: Tampons or pads (for mommy!) I got my postpartum period at 4 months & they are so unpredictable after pregnancy!
Tonya: Doggy poop bags. Great for storing a dirty diaper when there isn't an appropriate garbage can around. And they smell nice, so there's that.
Liza: In the first few months, I never left the house without puppy pads. It never failed in those early months that they would have diaper blowouts in the car seat. A [car-seat tech] recommended the pads to help. If they had a blowout, I could clean them up then put a pad down to keep the poo/pee off their clean clothes. I mostly used it beforehand (I knew when poop day was coming and prepared) to keep the car seat clean.
Erica: Lip gloss! Because I always needed a beauty boost in the throes of it all.
Liz: A nail file for myself. When I was nursing my nails would just break, peel, flake, crack so easily, even with vitamins. I used my nail file more than anything.
Michelle: Obviously extra clothes for the kid(s), but I learned to throw an extra shirt in there for me as well!
Marlene: I’ve tried to teach my daughter to keep a small bag of ‘what ifs' in the diaper bag for her twins: a Ziploc bag with a couple more empty bags in it, a small sewing kit, a roll of duct tape, a small pair of scissors, a small change purse with change and $1 bills. I like to plan, lol.
Vivian: I never forget a spoon, lol. Never know when baby wants a bite!
Inés: When they were newborns: clean undies for mommy! Those darn sneezes.
Source : http://thestir.cafemom.com/baby/208675/18_unexpected_must_haves_you