When nightfall comes, your only light is from the flickering flames and any flashlights or lanterns you brought. Without batteries to power the latter two, you're walking in the dark.
Avoid it: If this is your first time camping, make a master camping checklist well before you leave, this way you can add things as you think of them. As you pack, check off each item, including batteries. You can also create a camping box, and keep extra batteries in it so you never leave home without them.