Revealing mistake: In one of Pickett's charge scene the bayonets are obviously rubber.
Revealing mistake: There are white wristwatch marks on the tan arms of the Confederate soldiers as they reach for Lee as he rides among the troops.
Revealing mistake: A jet con trail can be seen just before the fight at Little Round Top, when Chamberlain and Tom are talking.
Continuity mistake: On the first day of the battle when Longstreet and Lee are talking, Longstreet's cigar is unraveling and in the next shot he has a new cigar.
Revealing mistake: Just before Pickett's charge, there are a number of shots of drummer boys playing the drums. Their drums have plastic drum heads with the "Remo" brand name on them.
Revealing mistake: In the battle of Little Round Top, as the Confederate soldiers are backing down the hill there is one man walking backwards and loading his gun. Behind him another soldier aims his weapon and fires, shooting the first man in the side of the head. The man then ducks and waits for the smoke from the shot to clear and continues down hill.
Continuity mistake: At the beginning of the movie it is dawn of June 30th, but should be dawn of 29th. Harrison the scout completes his task and reports to Gen Longstreet. Longstreet confers with Lee and it is now evening. The next several scenes show a new dawn when Sorrell asks if he should wake the men, Buster wakes up Col Chamberlain and a bit later Gen Buford arrives in town with his cavalry and no fighting yet. This second dawn should be June 30th, not July 1st, the first day of the battle. Further that day ends with Buford drafting report for Gen Reynolds.
Revealing mistake: There is a scene in which Union soldiers are standing on Little Round Top. You can see the General Warren statue, which is partially hidden by shrubbery.
Revealing mistake: In several scenes where the cannons are firing, they either don't recoil or recoil very little. In real life, they would have a bigger recoil.
Visible crew/equipment: Lights from what appears to be the high school football field are visible in the distance during Pickett's charge.
Revealing mistake: In the scene where Chamberlain is walking with his brother just after Little Round Top and before they get to the safe part of the battlefield, there is a jet con trail in the sky.
Revealing mistake: When Buford's cavalry is first riding into Gettysburg, he brings his men alongside a fence. On the other side of the fence, there is a trail with tire tracks from a car or truck. They can't be from wagon wheels because they are about a foot thick each too well formed.
Continuity mistake: After Chamberlain's bayonette charge, he sees his aide wounded for a second time. The scene changes angles back and forth during their talking of how brave the boys were. An American flag is over the shoulder of the aide on close ups, but on a longer shot the flag is leaning against the tree.
Factual error: During the engagement of the 20th Maine in defense of Little Round Top there is a tremendous amount of firing going on as they repel charge after charge of Confederates. Yet, there is not a single leaf, or tree branch, that falls to the ground. With that amount of lead flying around there would have been debris everywhere. There are actual Civil War accounts where whole trees were cut in half by bullet fire.
Continuity mistake: During the sequence at Little Round Top, the same footage is used twice. The 20th Maine soldiers discharge a volley of musketry and there is a shot of a Confederate soldier being bodily flung backwards. A short time later, the Union defenders send down another volley and we see a close-up of another Confederate falling. Only it's the same actor, same footage, from a different camera about 35 feet down the Union line.
Factual error: When General Buford looks back toward Gettysburg from the cupola of the Lutheran Seminary building to see if the First Corps is approaching, the director used a colorized photograph of the town taken by Matthew Brady a few weeks after the battle to illustrate Buford's view. Unfortunately, the photo was taken from a point north of the Chambersburg Pike and the Lutheran Seminary is south of it.
Visible crew/equipment: During Picket's charge, shadows of camera trucks can be seen a lot.
Continuity mistake: In the scene where Longstreet rides up to Alexander, prior to Pickett's charge, his staff stops shortly behind them, while in the next close up shot they have moved much further away to the right.
Continuity mistake: When the battle is happening at the end, the number of soldiers bodies after they have been killed varies from shot to shot. It shows them falling and dying in some shots, but the bodies are gone in later shots.
Factual error: There are several scenes which show an American flag with way too many stars for that time period.
Answer: Mainly because his soldiers were tired after a long march and he didn't think they could make a direct assault on an elevated position (the hill was not quite as "bare" as the film suggests). There were also reports of Union reinforcements arriving to the area, and Ewell was afraid that his army might be exposed and overwhelmed if he attempted to take the hill without assistance (Ewell had requested, and been denied, reinforcements of his own).