What is it called [when you make a mistake while trying to correct another person's mistake?]..

[rowrrbazzle]

For spelling and/or grammar, it’s Bell’s First Law of Usenet.

@Garfield226

… “Flames of spelling and/or grammar will have spelling and/or grammatical errors.”

This was first posted by 15 May, 1990, by Andrew Bell to alt.sex. (cite).