First, cake-baking rookies often follow a brand new recipe on the day the cake needs to be prepared. This often leads to disasters, especially if the recipe is from an unknown source. If a new recipe piques the interest, I highly recommend baking a practice cake beforehand. Even if the recipe is a legitimately good one, people have different oven heating efficiencies, and the temperature or timing stated in the recipe may be different with another oven.
Timing is one of the most important factors in baking a great cake. The most commonly made mistake is under-baking the cake. Under-baking leads to a cake that collapses inward at the center. Opening the over door to check on a cake can be equally detrimental because it results in a large temperature drop. If the baking was not yet complete, the cake will also collapse. This is especially true of flourless cakes, such as cheesecake.
Cutting corners is another common mistake often made by people who are baking in a hurry. For example, they might under-sift dry ingredients, or under-mix the batter. Cakes with under-sifted dry ingredients are often lumpy because the leavening agents were not evenly dispersed. I most often notice under-mixing in chocolate cakes because there are random swirls of white and black. Also, the texture of under-mixed cakes tends to be coarse because the butter and sugar were not properly creamed together.
While under-mixing can be disastrous, over-mixing is even more common. Over-mixing can result from either mixing the batter for too long, or mixing it too vigorously. This causes air pockets to form, and leads to a cake that has tunnels or holes. To avoid this, I only use an electric mixer to beat the butter and sugar for a few minutes. Then I gently fold-in the remaining ingredients by hand. It may take a little more effort, but following a few basic rules results in a cake that is moist, beautiful, and delicious.