Instead of correcting Keynes’s mathematics (which is right), the paper concludes that there has been a “slip of the pen” in his own description of these concepts. (Google)
Therefore, it is very inconvenient to use the conventional ballpoint pen if there is a slip of the pen which needs to be corrected or deleted. (Google)
According to Freud, there is a "psychology of errors"; that slip of the tongue or that slip of the pen become for Freud the clues to the secret functioning of the unconscious. (Google)