What makes a cake from scratch fail?

My nephew tried to make a carrot cake from scratch w/a recipe he got from ehow.com. He followed the directions exactly, with the exception of how he mixed the ingredients. He didn't have a mixer with him, so he just mixed it all up for a while. Would the consistency of the batter screw the whole thing up? It looked dark brown and/or black and bubbly. It started out looking like a normal cake in the oven, but something went horribly wrong. What gives?

Here are the ingredients:

2 c sugar; 1 1/2 c veg. oil; 4 eggs; 3 c raw grated carrots; 2 c flour; 2 tsps baking powder; 2 tsps baking soda; 2 tsps cinnamon; 1 tsp salt