It has happened more than once that a professor has assigned a textbook, which I bought, only for the professor to say in the first class that the only reason they assigned a textbook is because they were required to, but will never use it. Holding off on buying textbooks until after the first class (or, I guess, emailing the professor to ask if they plan on using the textbook) would have saved me several hundreds of dollars. (Having textbooks to study from is nice—they are, to me, the most efficient way of getting up to speed in math or science—but the ones professors assign because they need to put something down tend not to be the best ones.)