Frixion pens have taken the stitching world by storm with their erase-ability! These pens technically have an 'eraser' but in order to erase from fabric, you only need to apply heat (iron, blow dryer, etc.). It's always good to test first as on some fabrics this will leave a little bit of a ghost line (not a problem if you're stitching over it, however). Using this is a great way to map out your stitches as you go if you're practicing a more sketch-like style or wanting a specific look to your mending.
This particular pen is a fine (.5) line blue ink pen. It is clickable, so no cap to lose!