Early in the movie Harper Lee and Capote order drinks in a Holcomb, Kansas restaurant. At the time of the movie's action, Kansas was dry as a bone. You couldn't buy a drink in a Kansas restaurant until state liquor laws began to change in the mid '80s.
Perry's letter to Capote acknowledging receipt of the pornography closes with Perry's signature and address, which includes the ZIP code for the penitentiary. The ZIP code was not officially introduced until July 1963 and not widely used until some time after that date.
In a scene where Harper Lee and Capote are eating in Kansas, there is a menu with prices in the background. It includes a hamburger for $ 2.25. That is well above, more than double, the price of a hamburger meal in Kansas at the end of the fifties.
After watching Kitty Dean in the opening scene, Capote returns to his room to write some notes on a yellow pad. He first writes the date (11/16/1959), then the title "Answered Prayers". In the next scene, he throws the pad down on his bed, with a doodle of a man's face under the title. The date has disappeared.
Perry is depicted as being substantially taller than Truman when in reality they were roughly the same height. This may have been done purposely to give Perry a more intimidating presence next to the frailness of Capote.
Truman Capote never saw bloodied mattresses in the Clutter house when he visited the scene of the crimes. The mattresses, bedclothes, sofa and other bloodstained items were burned on Monday 16 November by four friends of Herb Clutter's who volunteered to clean up the house. Capote didn't arrive in Holcomb until several days after the Clutter family's funeral, at the end of that week.
When Capote first visits Hickok and Smith, Lowell Lee Andrews ("Andy") is seen occupying the cell next to Hickok. The movie is clear at several points that Capote's conversations with Smith and Hickok took place in the county jail at Garden City, Kansas before and during the murder trial. Hickok and Smith only became acquainted with "Andy" while all three were on death row at the Kansas State Penetentiary in Lansing. The three men were never incarcerated together in the county lockup, as "Andy" committed his murders hundreds of miles away from Holcomb in the town of Wollcott.