I have had many cookie disasters in my day. There are a number of things that normally trip me up.
1- Cookie recipes normally call for creaming the butter and sugar. I only got an electric mixer a couple of years ago, so before that, this was a major problem for me. I am not strong (or determined?) enough to do this properly by hand, so it never quite worked out.
2- I always keep my butter in the freezer and am never patient enough to get it to room temperature when I am making my cookies. This results in cold butter being whisked with the sugar. Don’t do this! Your cookies will be the completely wrong texture and you will regret it in the long run.
3- I usually cook them too long. Cookies often firm up quite a bit once they are taken out of the oven. I never realized that and just thought that baking times were completely off and cooked them until they were cooked through, which resulted in horribly overcooked cookies.
4- I drop entire trays of hot cookies on (the relatively clean) floor where they break into a million pieces (because they are still mushy and hot) and I have to salvage them by making a ‘trifle’ as the potluck dessert.