Using too much or too little salt
There’s nothing worse than under-seasoned food except for over-seasoned food. Salt is one of those ingredients that takes time or a recipe to master. Also, not everyone has the same tolerance for saltiness, but there’s usually a margin of tolerance that every person’s taste buds fall into.
That said, you’ll have to learn how much salt to put in meat and vegetable dishes from experience. Following recipes is a great idea, and you can always prepare a salt shaker for your dinner guests. With that in mind, it’s infinitely better to under-season foods than to go overboard.