I always have a spare outfit for my younger 2.I always have extra kid clothes stashed somewhere around the van. You never know!
I used to carry a spare outfit for my daughter, but she's almost 6, and I've been able to avoid doing that for short trips, like to the grocery store, etc. where-as the toddler/baby always has an extra. LOL
I keep a pack of diapers in my car. My oldest took all my diapers out of my diaper bag a couple of times with out telling me, so I learned to keep a stash hidden in the car.