When 2.6.21 came out, I decided to install new kernel headers. I learned that if you do make INSTALL_HDR_PATH=/usr headers_install over top of a finished system, it destroys everything in /usr/include other than what make headers_install thinks should be there.
This was expensive to fix. I think a note in the book not to do that would be a good idea. There are other means to this end such as installing to a temporary directory and cp -R; although it's also arguable that if you want new kernel headers, the only sane way to do it is as part of a fresh start.
-- Marty Jack
