“Do not rub out, if you can possibly help it, in drawings that aim at artistic expression. … it has a weakening effect, somewhat similar to that produced by a person stopping in the middle of a witty or brilliant remark to correct a word.
“If a wrong line is made, it is left in by the side of the right one in the drawing of many of the masters. “Harold Speed, The Practice and Science of Drawing, page 269
When is a mistake not a mistake but part of the piece?
I’ve found that using pen rather than pencil has helped me focus on moving forward through and along with ‘mistakes’, because erasing isn’t an option. Not that it always ends up in a satisfying place, but it’s taught me to work with what’s happening rather than changing my mind and starting a bit again as happens when working with an eraser.