When preparing the three defendants for trial, Col. Dax tells them that he's seen the room where they'll be tried, and that the afternoon sun will be in their faces. In fact, the sun is at their backs.
After the court martial, as the sergeant is addressing the guards describing the procedure and discipline required of the firing squad, all of the guards in the rank have a "710" regiment number collar pin whereas the sergeant (and those in Col. Dax's regiment) have a "701". Although it might be a simple matter of the costuming department transposing the two numerals, the difference might also be intentional. If a firing squad is going to execute three men in a given regiment, then you'd want to have men from another regiment come in and do it to avoid having men shooting their friends.
After Gen. Mireau slaps the soldier in the trench, he continues on to Col. Dax's dugout and three soldiers carrying a machine gun pass him. The same three soldiers still with the machine gun pass him again when he and Dax are looking at the Ant Hill through the binoculars.
The goof items below may give away important plot points.
Near the beginning of the film Pvt. Ferol, when asked by Gen. Mireau, states that he has no wife--but while walking to the firing squad he is crying on the shoulder of the priest that he will never see his wife again.
During the execution itself the sky repeatedly shifts between gray and overcast in some shots to bright sunshine in others, noticeably changing the natural light, causing shadows and sun glare to appear and disappear from shot to shot.