Let's say I was practicing my ball throwing for baseball. I consistently each day go out and perform the same rote task of throwing the ball from shortstop to first base. This is a common task to do. I get really good at it and really nothing else. Good I think, I've mastered a skill. But I am wrong. I mastered a skill but not a more difficult skill that is highly prized or needed. Also, let's say I have a slight flaw in my throw, the ball still gets there but it makes the ball move wildly once every ten throws. Well that's a big flaw that over time really hurts my chances of playing in the big leagues. No one ever really tried to correct my technique and now it is a habit for life. I could have really pushed myself to fix the technique and learned great skills.
This happened because I didn't follow the right advice about practice. I also just blindly thought that I was practicing and that in a certain period after prancing and passion for the game I should be the next great baseball player pulling in millions.
I was never into baseball that much to put in effort or time so this is all hypothetical but any sport I was playing, like tennis, never put in enough nor did I participate the way I should have to become a master. I should have followed the real advice that I must practice in a way to excel which is through pushing boundaries and fixing all mistakes.
And this can work in more than just sports. It can work everywhere. To become great you have to spend time in something and spend the right kind of time. One that will advance and not make you skillful in a low skill but introduce you to master a hard learned skill.