From Scientific American... It is still quite possibly simply a popular myth.
The last instance of trebuchet
use comes from the New World, at the
siege of Tenochtitlán (Mexico City) in
1521. As ammunition was running critically
low, Cortés eagerly accepted a
proposal to build a trebuchet. The machine
took several days to build, and at
the first launch the stone went straight
up, only to return and smash it. In view
of the tremendous power of these devices,
and the finesse required to make
them function properly, would-be replicators
should take careful note.
Here is the link...
http://www.deremilitari.org/resources/pdfs/trebuchet.pdf
Subject: Trebuchet Failure 
Tue Apr 20, 2010 8:44 am
