At least twice, Doc Brown says that 3,000,000 BC is in the middle of the Cretaceous period. In fact, the Cretaceous period ended approximately 63,000,000 years earlier, around 66,000,000 BC.
Doc Brown and his sons travel to 3,000,000 BC and accidentally prevent the extinction of the dinosaurs by using Doc's new invention to disintegrate a meteor.
Even in 1991, the extinction of the dinosaurs was recognized by the scientific community as having occurred approximately 65 million years ago.