First the extra ink on the "O" of postage seems to be either an extra drop of ink or a printed flake of thin paper which has affixed after the stamp was printed. The later suggestion is based upon what seems to be a pattern of the ink in area of question.
Next the term "error" has a broad general definition of just about any human mistake. Now what rogdcam was trying to explain is in USA philately and this is a USA based forum, the terms of Error, Freak and Oddity are terms of art in US Philately. As detailed review of they types of errors, freaks and oddities can be here:
https://www.efocc.org/Resources/Hot...AA_Cover.php at the website of the EFO Collector's Club. That listing was first developed in the late 1970s and was considered complete by 2000 for the stamp printing and production methods up to that time.
That said, I am aware that the term 'error' is frequently use to describe any fly speck or greater issue found on stamps of other countries, especially the British Empire matter. In US terms of art, yours is if an ink splatter is a freak. If post printing adhesion, damaged.