Why are my baking recipes turning out inconsistent?

The most common cause is inaccurate measuring, particularly of flour. Scooping flour directly from the bag packs it into the cup and can give you up to 50% more flour than intended. The correct technique is the "spoon and level" method: spoon flour into the measuring cup until heaped, then level the top with a straight edge. For absolute precision, use a kitchen scale and measure by weight instead of volume.