Bloomberg | "Acing the GMAT" | June 2011

"Tip: Imagine that a novice dancer learns a new step but practices it only once or twice before suddenly switching to a new step. How well do you think he or she will learn to dance? The answer, of course, is not well at all. Unfortunately, most GMAT students study sentence correction the same way, constantly dividing their attention among countless error types.

Unless your verbal ability is quite advanced, do not study entire sentence correction (SC) problems at once. Instead, pick ONE major error type, look for it in a large number of problems, and ignore other error types. Once you've mastered enough SC error types, you should progress to entire problems, but not until you've mastered the major individual concepts."

Top GMAT Prep Courses | Q&A With Manhattan Prep Expert GMAT Instructor, Ron Purewal | October 2014

If you’re not thinking the right way, though, the corresponding realization—“You know, I need to change the whole way I’m going about this”—is vanishingly rare.  The common reaction is, instead, “I’m going to do even more of what I’ve already been doing.”  Still won’t move the needle, and might even reinforce existing bad habits, making them harder to correct down the line.