I can remember coming in from school and my mom yelling, "Don't slam the door; I have a cake in the oven!" There are several reasons a cake will fall in the middle. The most common cause for a cake sinking in the middle is it is under-baked in the middle. You can tell that your cake is fully baked when a cake tester or toothpick inserted into the center comes out clean. Make sure your ingredients are room temperature, and don't over-mix. And finally, try your hardest not to open the oven door while baking because this allows cool air to hit your cake and can cause the cake to fall.