One day you’ll find yourself saying “real.” “Real” is an adjective, not an adverb, and since only an adverb can modify an adjective, “real good” and “real bad” are both incorrect.
Extra Points
Of course, you can use the adverb “really” instead, but you can almost always find better, single-word choices, e.g. “great” rather than “really good” and “awful” rather than “really bad.”