Factual error: Three of the Earp brothers arrived in Tombstone on December 1, 1879. They served as lawmen during 1880/81 and fought the Cowboys in the famous gunfight on October 26, 1881, They left the area in April 1882. In the movie the Earp's arrive in Tombstone in 1881 and ride past Boot Hill Cemetery. The tombstone displaying one of the most famous epitaph ever written is visible: "Her lies Lester Moore, Four slugs from a .44, No Les, No more." The problem is that Lester Moore wasn't killed until 1884. So it's impossible for his tombstone and famous epitaph to be in Boot Hill when the Earps arrived.
Factual error: In the scene when Virgil returns to the Oriental after being shot, Wyatt and Morgan are sitting at a table eating Chinese noodles with vegetables, among which is broccoli. Broccoli was practically unknown in the United States until it began to be commercially cultivated in California in the 1920s.