Most of the advanced math I learned used textbooks from the early 2000s era, which were often junk; this was the beginning of the "new book every year" phase of

Most of the advanced math I learned used textbooks from the early 2000s era, which were often junk; this was the beginning of the "new book every year" phase of universities, one which they seem to have never left. (What I did retain was usually taught by lectures or compassionate fellow students.) My multivariable calculus book was recalled by the publisher mid-semester! One of the more talented kids got so pissed off at its quality that he went through the whole thing and ended up finding several hundred calculation errors in it (not typos or typesetting).