Before Valdez shoots from long distance in the valley, Tanner's man Segundo says he has already killed 5 men. After the valley ambush, when Sugundo meets up with Valdez, he confirms there were a total of 11 killed. However, Segundo told Tanner there were 5 more killed in the valley, so Valdez should have said a total of only 10.
The photograph of Valdez with the 7th Cavalry has a several errors. The text on the photograph states it is of "Brig. Gen. Crook and Staff 7th Cavalry Troop A. Apache Territory 8 May 1883. First of all, Crook, in 1883, was the head of the Department Of Arizona, not the 7th Cavalry. Second, a Brigadier General would not have commanded a regiment (let alone a company or troop). Instead, a Regiment would have been commanded by a Colonel. Third, the 7th Cavalry was stationed on the Great Plains, and was never further south than the Indian Territory (present day Oklahoma), when it was at the Battle of the Washita in November, 1868. In 1883, the 7th was stationed in the northern plains.
At the end of the film, when Valdez is riding hidden between two horses, a wire is visible holding the horses' bridles together, so that they won't separate during Lancaster's close-up.
A rider galloping on horseback and firing a rifle is possibly the most ridiculous waste of ammunition portrayed in cinema; any hit would be extremely coincidental.
Valdez has stored his Sharp's carbine in a loosely-wrapped, dusty bundle for a period of (presumably) years, with no special protection, yet when he draws it forth, it appears to be in pristine condition.
In the picture of Valdez, taken when he was an Army Scout, he had Sergeant stripes on his shirt. Later, when he wore the shirt, there were no stripes.