I've been lately quite active on #debian irc-channel on oftc, and http://forums.debian.net
More and more new users come asking help, when something does not work, and very soon we discover, they have added 3rd party sources to /etc/apt/sources.list There is an excellent article on Debian wiki: https://wiki.debian.org/DontBreakDebian which we on those forums suggest them to read on these situations. I've been wondering, many newbies maybe woulnd't have to reinstall, it they had read the article soon enough. Could there be a link to that article in some doc so it would be easy to spot? Arto Keiski