A great quote from the writings of Herbert L. Clarke. It is amazing how much of what he says applies to learners today:

In my practice I kept to the elementary, although I could play a lot of tunes when I first started and this even before a perfect scale was played – that is, played without making a mistake of any kind. How often do we think that our work is satisfactory when, after all, we merely blow into the cornet and make a noise without being perfect in every detail! One hundred percent alone is perfection. Ninety-nine percent only proves that one percent is missing in perfection, thus making the whole imperfect by just one per cent; therefore, when in his practice a player does not correct the slightest mistake immediately he logically is practicing to be imperfect.
I have heard many pupils play page after page of the instruction book, missing the notes here and there and making all manner of mistakes without correcting them, then say: – “well I played fifteen pages of exercises today.” There was no realization that even if only one mistake was made they had not played the fifteen pages, but simply “played at them.”