Using eggs straight from the fridge
Your eggs should always be at room temperature before you use them. Cold eggs prevent the mixture from aerating properly, which means the texture of whatever you are baking will be heavy and dense.
You soften butter too much - or not enough!
Butter that is too soft won't hold air, giving you a dense and heavy mix, but so will butter that’s too hard and cold. The best way to get perfectly softened butter is to let it sit out at room temperature for about 15 minutes. It should give a little when you press down on it, but it shouldn't break, crack, or lose its shape (and it definitely shouldn’t turn to liquid).
Your baking powder or sodium bicarb is old
Sodium bicarb and baking powder act as leavening agents in the baking process, helping to give baked goods their rise. If it’s old and stale, it won’t work properly. Buy it in small quantities and replace it every six months.
Overmixing
Are you an overzealous mixer? Mixing flour with liquids helps create the gluten structure that we all know and love. But overmixing can lead to the development of too much gluten, making the result of your baking tough and chewy.
If a recipe warns you not to overmix, mix until there are no more visible streaks of flour or dry ingredients – then stop!
Over measuring
Spoon your flour into the measuring cup and scrape off the excess with the flat side of a knife. Don’t pack the flour down.
Baking on the wrong rack
To get the most even bake, use the middle rack of your oven. This is where air is circulating, and heat sources are evenly distributed. If you have more than one tray baking at once, be sure to switch them halfway through.
Incorrect temperature
Make sure your oven is properly preheated before putting in whatever you're baking. If your oven is not quite at the right temperature, it can alter the cooking time and texture of your baking
Too many sneaky peeks!
Heat escapes every time the door is opened, so it's best to use the oven light and a glance through the glass door to check on progress.