The Outlaw
This Howard Hughes film was blocked because the primary directive of the director was to showcase as much of Jane Russell's boobs as possible. She didn't really appreciate that, and neither did the 1943 Hollywood board of approvals. Once Hughes cut one minute of content (all breast-related), the movie was given the OK.
Titicut Follies
This strange documentary was intended to look like something it isn't in order to attract public attention. Unfortunately, this movie (which is about the terrible treatment of the criminally insane inmates at the Massachusetts Correctional Institution) was banned because it "violated the rights of the patients," despite the crew recording their consent before the filming happened. It's assumed that the Massachusetts Correctional Institution was probably just terrified that the American people would object to their terrible treatment of the mentally ill.