There are a lot of really crazy conspiracy theories out there. Anyone who holds such beliefs is equally loony and likely needs mental help. At the very least.
My favorite conspiracy theories (your mileage may vary):

** Sept. 11 attacks were an inside job
** The moon landings were faked
** The Holocaust never happened
** (insert any politician's name here) is a space alien planning to control the earth
** (Hitler, Elvis, JFK, John Lennon, etc) faked their death and are living in obscuring somepace
** Obama was born in Kenya