The arguments of 9/11 “truthers” on the impact of American Airlines Flight 77 into the pentagon are particularly farfetched given the existence of multiple videos of the crash and hundreds of eyewitnesses as well as massive physical evidence including large parts of the plane with serial numbers and DNA identification of the remains of the passengers.
I think these two videos are particularly effective in debunking their claims. The first features all three live surveillance videos of the crash, which although they are at a low frame rate clearly show parts of the fast-moving plane as it impacts the building. The second is a nice, simple explanation of how truthers use quotes out of context to misrepresent the statements of key eyewitnesses. Why they engage in deliberate misrepresentation of the facts is a question which enters the realm of abnormal psychology.