Cash. Many of the cards and id's in my wallet, so the whole wallet goes in the pockets. Eyeglasses for work. Jacket or vest, gloves that were needed in the AM but is too much in the PM. Pager. Sometimes a nice sub sandwich. These things end up in my pockets frequently, so that there is less room for the tools and flat repair stuff. That's why I leave that stuff in a seat bag. But I use my road bike to travel to work, so that means I end up bringing a lot more with me than on a pleasure ride.

I have forgotten everything on that list at least once, many more often than that. That is why I like all my flat repair stuff on the bike, where it isn't dependent upon my memory. But I could fit my seat bag in my jersey pocket, when I am not filling them up with stuff, to achieve the effect you are looking for.