Some of Raza's men are shown wearing shirts that button all the way down the front. These shirts did not come into common usage until the following decade after the film's setting and even then were too expensive for poor villagers or bandits to commonly wear.
While Rico is shown to be a weapon's expert, unless he had spent some time in Europe prior to WWI or during the early portion of the conflict, there's no way he should have been familiar the French 75mm howitzer. The weapon was not commissioned until 1911 and its first wide-scale use was on the battlefields of Europe during the war.
The weapon was sold to the US; however that was later during the conflict and after WWI, thus after the setting of the film.
When the team makes it back to the train after capturing Mrs. Grant Claudia Cardinale and the ambush is waiting for them, Ehrengard Robert Ryan is initially shown from inside the train car sitting facing sideways to the open door of the train car, then from outside he is shown with his back to the door of the train, then back to inside and he is sideways again.
At the end of the scene when they are sitting on a ridge line talking about battles and promises, Farden exits the shot. Dolworth puts his cigar in his mouth and puts his binoculars to his eyes backwards, small lenses out.
When Fardan (Lee Marvin) has the Dolworth (Lancaster) and Jake (Woody Strode) synchronize their watches at 4:30, Dolworth states that the fuse that will detonate the water tower at Raza's compound will take exactly a half hour. The time for the detonation is set for 5:00. However no time was taken into account to have him set the charge, which had to take at least a few minutes.