Overmixing

For baked goods that rely on leavening agents like baking powder and soda (muffins, brownies, many cakes, quick breads, pancakes) the liquid ingredients and the dry ingredients are mixed in two separate bowls, then carefully folded together.

It is imperative not to overmix, as this creates a tough and dense product, and the dreaded tunnels...

I first heard about tunnels in an eighth grade home ec class. Our teacher tore open a muffin and pointed to round, gummy holes throughout.

"The morning class didn't manage to avoid the tunnels, let's see if you do." (We didn't.) 

Tunnels, she told us, were created by over-mixing the batter. Basically, the more that flour is mixed and worked in a batter or dough, the more strands of gluten are formed. Hot air travels up through the strands creating tunnels, which create a gummy, dense structure. To avoid this, the batter must be mixed sparingly. It should remain lumpy, with only enough stirring to moisten it and streaks of flour preferable to additional mixing.