This is not uncommon. Some needles go through an oxidation process and will turn black (this is a result of the nickel plating wearing off). It is important to change your needles frequently to keep this from happening! You could also consider the purchase of platinum needles.
It's a simple tip, but it's so very important to wash your hands before you begin any needlework project. Your hands may not look dirty, but over time the oils on your hands and skin can really affect the condition of the piece that you are working on.
If you find that you have made a mistake in your work and have to remove stitching, NEVER reuse the thread. The friction of having to remove the threads damage the fibers and will give your finished product a "fuzzy" finish!
As soon as you see that you have made a mistake, stop and remove the stitches before continuing on your project.
It is simple to remove a few stitches. Remove them by inserting an unthreaded needle under the stitches one at a time and gently pull them out.
If you have already worked on your piece and realize you have a mistake in the middle of a pattern, carefully cut them away and restitch the correct stitches in their place.
It's unfortunate, but sometimes it happens. It's easy to repair small rips and tears in your canvas. Cut a small piece a few meshes larger than the piece that you will be repairing, this will be the "patch". Line it up with the canvas on the underside and baste in place. Carefully continue stitching. Cut away loose ends that remain on the top side of the canvas.
Did you know that needles have a bur? Because machines punch the eyes of most needlepoint needles they will have a small bur on one side. This problem is easily corrected by unthreading your needle and rethreading it through the other side!