What is a common result of excess mixing when preparing dough?

Excess mixing when preparing dough can lead to a tough product and the formation of tunnels due to overdevelopment of the gluten structure. When dough is mixed too long, gluten strands become overly elastic and can cause the dough to become dense and difficult to rise properly. This overmixing also entraps air in a way that can create irregularities within the baked product, resulting in tunnels or holes. The tough texture is a direct result of the gluten being too developed, which restricts the dough's ability to expand and results in a less desirable final product.