I always thought it would be nice to have a stitch eraser but never felt that I needed one. Now, after trying one out, I can see it would be a worthwhile investment. See Ann's Stitch Eraser here.

A stitch eraser is one of those items that you don't need, until you do. Then it is invaluable.

This one is cordless, which is convenient. It also has a narrow blade which allows you to get into tight stitch areas without chewing up a lot of fabric.