During one scene at an enemy check point, the night sky is full of stars (2:23:55). We see a mounted enemy patrol riding by the check point under the moonlight (2:24:02). A few moments later, outside is clearly daylight (2:24:45), yet when the enemy soldier inside the check point booth blows into the lamp (2:25:29), the booth is in total darkness and there is no daylight coming through the windows.
Pilar and Pablo use an 1892 Krag-Jorgenson (or variant). Whilst it is possible, since the Spanish Civil War was called by one historian "A gunrunners paradise where they sold every type of firearm to the Loyalists", it is extremely unlikely that such a rifle, with its odd cartridge, was imported into Spain for the war effort.
Early in the film when Gary Cooper's character Robert Jorden meets General Golz, Cooper's shadow can be seen on a wall in the background. In the straight-on angle, it's Cooper's shadow, but in another angle it's obvious another person was used to create the shadow. When Cooper places his hand on his chin, the shadow's move is late by a second.
(at around 50 mins) Pilar says "Wait" to Jordan and Maria. It is clear that the shot has been reversed, as the bolt handle and magazine on her Krag-Jorgenson carbine (see previous entry) was on the left of the rifle, whereas this weapon was only made in right-handed versions.