Tough, Dry Cake or Muffin?

Sounds like a gluten problem. Gluten develops when you beat or mix a wheat-based mixture too much, making it stretchy as if you were making bread dough. Unfortunately, this is not what you want for cakes. Instead of a tender crumb, that extra gluten will give you a tight, unpleasantly chewy baked good. A side effect of having more gluten in the cake is that it will also tend to squeeze out or absorb more liquid, leaving the baked good in question with a drier interior. If you’re ever unsure of how much to mix, just assume that for cakes, less is better.