Continuity mistake: As Koulikov is shot while jumping the gap we see the blood spatter created lies along the dark line above the red writing. In the next shot it shows the spatter from another angle in which we see that it is now above the dark line where it was originally. (00:58:30)
Revealing mistake: Midway through the movie, there is a long-shot of several characters walking outside between two gutted factory buildings. The camera pulls back, zeros in on the base of a huge chimney and pans upward to the very top - where two men in black and white checkerboard jackets are working.
Factual error: When Kruschev and Vasy go to the press meeting the Soviet National Anthem is playing in the background. The actual anthem was not written until January of 1944, a year after the Battle of Stalingrad. In addition, the song sung is the 1977 lyrics. The lyrics used in 1944 were slightly different. See http://www.skazka.no/anthems/ for more info.
Revealing mistake: When the character played by Joseph Fiennes is in the Soviet headquarters, you can see his glasses shine with a purplish light. This is because his glasses are anti-reflex, something that was not available until much later than WWII.
Factual error: In one scene, a Soviet T-34 tank is shown in the middle of the battlefield. This is a T-34-85 model (as evidenced by its large three-man turret), which did not appear until late 1943. During Stalingrad, the Red Army was still using the older T-34 models which used a 76mm F-34 Tank Gun housed in a smaller two-man turret.
Continuity mistake: At the beginning of the film when the Russian troops are rushing headlong at the Germans, there is an obvious cut in one shot as several people shift places and one Russian just disappears.
Factual error: The German army never had a 116 Infantry Division in WWII. There was a 116 Panzer Division that was formed in 1944 from the 16 Panzergrenadier Division.
Continuity mistake: When Vassili is behind the chemical factory waiting for the Major to appear, the soldiers run out from under the train cars. The next shot shows the soldiers from behind, and if you look closely a horrible editing mistake was made. Some of the soldiers suddenly disappear, and then reappear.
Revealing mistake: When Danilov and Kruschev are in the war room, when they think Zaitsev is dead, pay attention to the maps on the walls. These are modern US and British defence maps that weren't available until starting about 1960. The clue is the the numbers on the squares. These are flight altitudes clearances for jets. Also, you can see that the maps are in Roman letters. Soviet maps during WWII were in Cyrillic and they looked differently.
Continuity mistake: During the scene where Vassili is hiding from Major König behind an old oven and trying to get his gun back by tossing his pocket knife at it and pulling the string, König shoots the string. The string breaks, but the bullet doesn't hit anything behind the string, as if it was stopped by the string!
Continuity mistake: Before they attempt to trap König with the raised helmet trick, we see one of them light a cigarette. It initially appears to be very large and misshapen. When the angle changes, the cigarette has shrunk to more than half its previous size and is perfectly rolled.
Continuity mistake: When Vassili and his two friends go out to trap König by shooting telephone repairmen, their rifles are covered by hessian, when they are shooting the repairman the guns are uncovered, and when Koulikov and Vassili are on the way back and jump across the gap in the buildings their rifles are covered again.
Factual error: Throughout the whole movie, Russian sniper Vassili Zaitsev is using a 3.5 power PU scope on his sniper rifle. In reality, Zaitsev used the larger, 4 power PE scope.
Factual error: When Commisar Danilov is dictating a letter to Vasily he spells out the word "honor" including the letter "h". There is no letter "h" or any equivalent in the Cyrillic alphabet or Russian language.
Factual error: Sometime in the movie somebody (I think the boy) says the Nagant rifle that Jude Law uses has a 4 times magnifying scope. When you compare the distance that objects have in the movie without scoping and through the scope of the rifle the magnification is way bigger than just four times. I know that cause I own a 4x scope myself.
Visible crew/equipment: After it has been discovered Sacha is dead, Tania is arguing with Vassilli. He is trying to prevent her from running to him. The camera gets close to the action and the shadow of its operator is blatantly visible on the actors.
Answer: For the same reason Marlon Brando and Gene Hackman were top billed in Superman. He was a bigger name and his agent was able to negotiate a better deal.