I know this is a strange way of putting it, but no, it’s not a mistake. In fact,I think it happens fairly often.
When practicing, there are likely to be difficult phrases that you stumble on and have to practice over and over again until you get it right. Usually this is done at the same speed. After playing this phrase 4-5 times, you finally get it right, then move on. Sound familiar?
There is actually something very wrong with this picture. The reason is, although your brain is able to distinguish between playing something correctly or playing something incorrectly, your hands – which do the actual fingering – are not. Your brain may remember how something sounds, but your hands remember only the muscle movements they repeat.
In the above scenario, what your hands remember is the 4 or 5 (or much more!)times it took you to get the phrase right rather than the 1 time where you actually did play it right. The more times your hands repeat a motion, the better they remember it, whether that motion is harmful or beneficial to your playing.
Thus,if you play something wrong many times and then correctly only a few times during any given practice – even though your brain may be happy you finally got it right – the next time you practice it is the mistakes that your hands will be likely to remember rather than the successes. This reinforcement of bad habits is what I call “getting better at playing badly.”
The way to avoid this trap is to play these problematic phrases slowly enough so that you can play them correctly after relatively few repetitions. That way,the reinforcement from the incorrect playing will no longer outweigh that of the correct playing, and your hands will remember the right movements the next time around.
Once you have reached this point, it is a relatively simple matter to increase the speed of your hands to where you want it. What is NOT a simple matter is fixing bad physical habits once they have become ingrained through repetition.
Of course, we all want to practice new techniques and phrases as fast as they should be played during an actual performance. Holding back is no fun, and requires discipline. However, this is actually a shortcut to mastering difficult phrases, and keeping your hands as free as possible from acquiring bad habits.