Over-stirring muffin mixtures
The easiest way to tell if you've over stirred muffins, quick breads or cakes is the texture when it's baked.

Correct, and it's all even.  Over stirred, and you'll have a series of larger bubbles in the cake, called 'tunneling', where it looks like worms have burrowed their way through your cake or muffin.

Stirring develops gluten, which is essential to trap in bubbles for most yeast breads, and to give it a little bit of chew -- but not something that you want in a typical quick bread. (muffins, cake, etc.)