Not using fresh baking ingredients
Go to your fridge or pantry right now and check the expiration date of your baking powder, baking soda, cream of tartar, and other common baking ingredients. Chances are that the product long ago, at least if you’re not a regular baker. Now, even though expired baking goods aren’t exactly toxic (repurpose old baking soda as a fridge freshener), you should still avoid using them.
Old powders may not provide the emulsification, the lift, or the spread that you’re looking for. This also holds true for old flour, although this is a lot less common of a problem. In conclusion, replace your expired baking ingredients right now!