Besides leaving things on and forgetting about them, it would be adding before tasting. Always taste or at least inhale deeply before making a significant change. Luckily, the veggies-in-the-pan, rice-n-bean types of cooking are so flexible that I can usually make a pot of rice or something to offset the salty/spicy. This is why I haven't yet gotten into baking all that much, vegan or otherwise (and am nonplussed by those who claim to be more comfortable baking than making stir fries or whatever). Cooking is always chemistry, but while vegetables cooked too long can result in less superior flavor, similar mistakes can completely ruin loaves of bread or cakes. lol

My mom (who's self-taught and has cooked for several decades) says a very common trend for new cooks is to add too much of things- going w/ the philosophy of, if the herb tastes good, then using lots and lots of it should be even better. You learn to go simpler and simpler as you progress, not unlike a writing style becoming less wordy and more concise. When I was first experimenting I would indeed throw about every spice and herb into the pan. This was a lot of fun (and healthy), but flavors are less likely to stand out this way, and can end up overpowering the dish. I've learned recently that, although vegetable oil is healthy and in some cases can taste amazing when used en masse (frying an oninon, for instance), for vegetables to brown or get a little crispy, less is more (the same goes for stirring!).

You also learn that not every random recipe you concoct out of things found around the house is going to turn out delicious. You have to use common sense (after all, your experiences with other people's recipes will tell you what will probably work in the future)- this was great in that recipe, why not use that element and add pasta, or ketchup or whatever. Throwing random ingredients into a pot can lead to disaster (needless to say, my mung bean/sweet pea/rice noodle creation did NOT go well).