When making chocolate chip cookies, I forgot to add white sugar while I was creaming the butter and brown sugar. I creamed the butter and brown sugar, had just cracked the egg into the mixture when I realized that I forgot the white sugar. So I added it and mixed it in along with the egg, mixing it a little longer than normal. I proceeded as normal with the recipe. The dough is currently refrigerating overnight. What effect will this mistake have on my finished 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.
There will be no strange effect. When you make cookies the important thing is that you mix the eggs and the sugar before adding the flour. Otherwise, if you had to separate the yolks from the whites, and you added white sugar while you were beating the whites until soft peaks form, it's normal that you have to mix for a long time.