The entire foundation of the conspiracy movement relies on the manufacturing and planting of evidence but I have yet to see any evidence whatsoever to support this allegation and when there's no smoke there's no fire.
JohnM
Offhand - and not really directly related to your question - we have the so-called "Hunt letter" that Oswald allegedly wrote to either E. Howard Hunt or, more likely, the Texas millionaire H.L. Hunt. The HSCA concluded that it was likely a forgery and a Soviet KGB defector said it was created by the KGB.
And the "Pedro Charles" letters that connected Oswald to the Cuban DGI agent Pedro Charles. Hoover and the FBI concluded that they were a hoax and probably done by an anti-Castro person or persons in an attempt to implicate Castro in the assassination.
The latter incident has been used by some conspiracy people to claim that the CIA - or elements of it - were behind the letters and is evidence of a conspiracy.