I recently came across this post and I thought it was perfect to share as I teach music and a lot of my students will try to do something they are learning and when they get it once or twice they start to speed it up and then they mess up and wonder why. It's MUCH better to learn something SLOWLY and play it perfect than it is to play it fast and very sloppy. This pretty much says it all:
Just something to keep in mind whenever you are learning something, whether it be music or anything else for that matter...