Following on from Erica McPhee's topic "How to make Corrections" from a couple of years ago. I'd like to suggest this solution.
I've used this simple technique on an important one-off scroll for presentation with success with an invisible repair.
If the word "year" for example is written by mistake as "yaer" write ea over ae. Allow to dry completely and with a new scalpel blade carefully scape off the parts of the original wrong letters which show. With a soft eraser rub over the new letters. If carefully done, the correction is impossible to detect.
In the example I cited, I was near the conclusion and saved many hours of rewriting with this simple method of correction.