When Sefton is explaining how he got hold of the telescope, he explains that he got together a "few lenses and a mirror". The telescope he has in the barracks is a refractor telescope, which doesn't use mirrors.
As is common in many older World War II movies, the SS men sent to transport Dunbar to Berlin are wearing the iconic black SS uniforms that were discontinued as duty wear in 1939, 5 years before the setting of the film. Additionally, the uniform of one of them has a cuff title with just the numeral "2." Cuff titles were only worn by combat units, such as the Waffen-SS, and contained unit names and/or symbols. Worn on a general duty SS man this cuff title makes no sense and refers to no known SS organization.
When Commandant von Scherbach enters the barracks and asks Lt. Dunbar for his serial number, Dunbar pulls out his dog tags and reads it as if seeing it for the first time. In reality, every serviceman knows his serial number by heart, and remembers it for life, as well as he would remember his Social Security number or date of birth.
When William Holden's character is laying on his bed on his back and first notices the swinging light bulb's shadow near his feet, the next scene shows the light from his bunk and there is no light source behind the swinging light that could have cast that shadow.
When Cookie starts to remove the radio from the pants leg of the soldier on crutches, it's visible right at the hem, but when the camera cuts away and then cuts back a moment later, Cookie is reaching farther up the pants leg to remove the radio.
At the movie's beginning, it says that at least the two escapees will have the longest night of the year (December 21st). Towards the end of the movie, it's Christmas eve (December 24), yet at least two weeks of events have elapsed since the movie's beginning and the end.
The length of the lamp cord for the mailbox signal changes in many scenes. When we see it "knotted" and Shultz pulls it down the first time, it sets less than a foot over the table. Then in another scene, it is hung closer to 3 feet over the same table. In the next mail drop, it again pulls down very close to the table. Then after the information about the "time bomb" is delivered, it now hangs higher again.
Just before Sefton reveals the spy, he throws an open jackknife onto the table and says, "Here's the knife to do it with. Only make sure you got the right throat." The knife quivers and barely sticks in the table. Shortly after, the knife is stuck firmly in the table, more upright.
Stanislas "Animal" Kasava is falling on his butt into the mud, but his white underwear isn't getting dirty. Two minutes later, running to see the Russians girls, he falls again and then he's muddy all over.
Again with the lamp cord Goof, in the name of "cinematic invention", Sefton sees the bulb and cord in silhouette on a brightly lit wall, yet later scenes reveal no rear light source, window, etc. able to cast the shadow.
Also, when Sefton leans back on the bunk, his head casts a shadow on the bunk post in the opposite direction.
The partial moving shadow of the dolly camera is visible for 1-2 seconds in the lower left hand corner as the dolly camera shoots it's close up of the lieutenant in the water tower in the daylight, before the scene cuts to night time.
It's December 1944. Every morning at 6:00 it's roll call for the prisoners of Stalag 17. Although in the middle of December, in southern Germany the sun will never rise before 8:00; the roll call in the movie is in full daylight.
Schulz is identified as a Feldwebel or Sergeant, but he is wearing the rank insignia of an 'Unteroffizier' or Corporal. The German Army's rank insignia were on the shoulder straps. A Feldwebel's insignia would be 'lace' that went around all edges of the shoulder strap, plus a star or 'pip' on the strap. Schulz's shoulder straps do not have lace at the bottom of the strap nor do they have have stars; that is the insignia of an Unteroffizier.
It was routine policy in the Third Reich to send captured Jewish enemy soldiers to extermination camps. This being the case, it's rather implausible that one of the prisoners in Stalag 17 is named Shapiro.
Price says he was with the 366th Bomb Squadron of the 35th Bomb Group at Chelveston. The 366th belonged to the 305th Group at Chelveston. The 35th Group flew antisubmarine patrol from British Guiana. In addition, Lt. Dunbar states he was with the 92nd Bomb Group at Waddington. Waddington was an RAF Bomber Command base. The 92nd BG was based at Podington.
The flight jacket worn by the captured gunner with Lt. Dunbar is not authentic with its slash pockets. Also, while the 600+ men in that part of the camp are all described as captured airmen, many are wearing infantry men's long wool overcoats and lightweight waist-length jackets that aircrew would not have worn. Aircrew would have some type of leather or cloth flight jacket or coveralls on rather than overcoats. Sefton's B3 jacket is authentic but looks almost new. He also wears an A2 in one scene. Not too unusual since gunner would wear many layers of clothing in their unheated bombers.
In at least two scenes, German solders are seen using US Browning 30 cal. machine guns; some still think of it as an error, but the use of captured enemy equipment was common by all sides in the war. A POW compound would be the ideal place to locate captured weapons, with a relatively limited ammo supply, whilst they still served to deter escape.
The map of Germany in von Scherbach's office in 1944 would include, not only Austria and Sudetenland, but also Gdansk/Danzig and the Polish Corridor, large parts of western Poland and the Saarland, all considered ethnically German by the Nazis and incorporated into the Reich. This could simply be an obsolete map never discarded.
We learn from the escape plan in the beginning of the movie that the Stalag 17 prison camp is located on the river Danube near Linz, which is on the Austrian and German Border. Later in the movie, when the prisoners are watching the women in the Russian compound, Cookie claims that on a clear day, you could see the Swiss Alps with this telescope. Nobody could see the Swiss Alps, with even the best telescope from this point of view, because the Austrian Alps would definitely be in the way. This could be a simple exaggeration of the power of the telescope within the character of the prisoners whose scientific knowledge is limited. With a good telescope one can see mountains on the moon; no telescope can ever show anything beyond the horizon.
The goof item below may give away important plot points.
During the smokescreen plan to rescue Dunbar from the SS, an improvised smoke bomb is hidden in an amputee soldier's pants leg. It slips out and creates smoke. However, in the wide shot of the commotion, smoke can clearly be seen from several smoke bombs on the ground, not just one.