Forgot to add sugar to cookies
It depends on how well you creamed the butter and brown sugar.  The point of creaming fat and sugar in cooking making is to put lots of tiny starter bubbles in the dough which will expand during cooking.  

If you really creamed the heck out of the butter and brown sugar, you might not notice a difference. (The cookies might be a little chewier than normal, not because of the over-abundance of brown sugar [you got the white in there eventually] but because of the extra mixing later.)  It would take a lot more time to get to a truly creamed stage - light and fluffy, where you can't feel any sugar if you rub the mixture between your thumb and finger - with half the sugar granules.

However, if your fat-sugar mixture wasn't creamed well, then the cookies will be flatter (less airy) and denser than normal.