Everybody does it (even though they shouldn't). Here are some tips for doing it well.
Many recipes for baked goods—pancakes, biscuits, and cakes in particular—call for buttermilk: Its acid reacts with baking soda to create a leavening effect, its viscosity gives batter a specific consistency, and its mild sourness contributes flavor. A common replacement for buttermilk is clabbered milk, which is milk that has been mixed with a small amount of lemon juice or vinegar. The acid provides tang and causes the proteins to aggregate, but it doesn't give the liquid body.
But when we made drop biscuits and buttermilk pancakes using clabbered milk, the results were disappointing. Clabbered milk isn't nearly as thick as buttermilk. That meant that both baked goods spread more than they should have, slumping into squat, uneven shapes. Cakes containing clabbered milk fared better since the batter was corralled by the walls of the pan.
We had a hunch that yogurt, another acidic dairy product, might work well in all applications. It was too thick straight from the tub, but once we thinned it with water, it was more or less identical to buttermilk in acidity and consistency, and baked goods made with it were hard to distinguish from those made with the real thing. We thinned whole or low-fat yogurt with an equal amount of water (e.g., ½ cup yogurt to ½ cup water), while thicker Greek yogurt called for a 1:2 ratio (e.g., ⅓ cup Greek yogurt to ⅔ cup water).
These can be swapped for each other in equal measures in most baking recipes with good results, but since sour cream has more than four times the fat, expect cakes and muffins baked with yogurt to have a slightly drier texture. Flavored yogurts such as lemon or vanilla can be substituted for plain in recipes where the flavors won't clash.
To Replace: 1 cup sour cream
To Replace: 1 cup plain yogurt