Doctor Who: 20 Things You Didn't Know About The Day Of The Doctor
6. Peter Cushing Movie Posters Were Cut Due To Copyright Issues
Doctor Who first made the jump to the big-screen in the 1960s, when Peter Cushing (most famous for playing Grand Moff Tarkin in Star Wars) played a character called "Dr. Who" in two movies: Dr. Who and the Daleks, and Daleks' Invasion Earth 2150 A.D.
These movies aren't official parts of the show's canon, but since The Day of the Doctor was a huge party everyone was invited to, Moffat still wanted to acknowledge them.
During the scene where Clara and Kate enter the Black Archive, he initially wanted some Peter Cushing movie posters to be included as background easter eggs, but unfortunately, copyright issues meant that this wasn't possible.