It's true while performing you don't get to start over, but I think it's good to start at the beginning, and go until you mess up again when practicing. That's what I do when I'm learning something new, whether it's a new lick, or song lyrics. Practicing is totally different from performing, because you don't practice what you know, you play what you know. You practice what you're learning, and try to put it in the play category. It may seem like a waste of time to start at the beginning, but I think it just helps you learn better. It won't be long before you play it all the way through, and you'll know you played it right. I think that is much more rewarding than working on something for hours, and then going back and trying to play it all the way through. It makes more sense to keep going back to the start, until you get it right. We are talking about beginners here, and as you progress you will learn how to cover up mistakes, and keep on going.