This article is from the Pratchett Newsgroups FAQ, by Orin Thomas firstname.lastname@example.org with numerous contributions by others.
There is *NO* Missing Chapter!
If some guy comes up to you at a meet and says "hey baby, wanna
come back to my place, see my local area network and download
the missing chapter?" he probably just wants sex.
One of the more recalcitrant threads that pops up, much like the
legend of the Yeti, Yowie or Bigfoot, is the legend of the Missing
Chapter. The story goes that, late one night whilst working on a
particularly hilarious section of the Discworld Chronicles Terry
accidently transmitted a section of his work to alt.fan.pratchett
and then had his computer suffer something akin to a complete hard
disk failure. Fun is had by all as alt.fan.pratchett regularly mounts
search parties around the internet looking for this particular untamed
ornithoid. (this was Leo's Fault! (and TP went along with it!))