Always a popular question along with "Do you have to start all over again?". At some point every calligrapher makes one - a MISTAKE! Yikes! So what to do?
I try my level best to avoid them. I am very fortunate in that the majority of my commissioned work is on manuscript calfskin vellum which is much easier to correct mistakes on than paper. It should be possible to make a correction on vellum so well that the naked eye will not see the correction. Paper is a bit trickier as the fibres can be worked loose (raising the nap too much) and the ink may then go on to bleed even after treatment.
I really do make very few mistakes - time is money - but I did make a mistake recently. I left a painted letter out of a word which was painted in Scarlet Lake - and all reds have a propensity to smudge. Fortunately I had sourced some fabulous erasers from an American company some years ago called SicPress. I have 3 which range from coarse, medium to fine and they are the best I have found yet. Working with a scalpel and ink rubber can be quite vicious whereas these erasers work like a dream - not only removing the paint but they don't "raise the nap" too much. After the paint or ink has been completely removed a light burnish with a bone folder over some crystal parchment followed by a small application of powdered gum sandrac and Bob's your Uncle! The result on paper will never be as satisfactory so I really am lucky I mainly work on vellum!