In the season 2 episode "What's my line (pt 2)" The Scoobies are in the library discussing what attacked Xander and Cordy. Xander explains "This was a man *of* bugs, not a man who *was* a bug." and slams a book shut on one of the bugs, the book is right in front of him and you can clearly see that it is a white biology textbook, however when the camera cuts to a shot of all of them standing around the table the book is gone, replaces by an old book with a red cover.