Goofs

The amount of whiskey in Klink's glass changes at least twice.

Hogan says that it is OK if the Luftwaffe inspects the P-51 since it has a German engine. The engine was not what made the P-51 such a great fighter, it was the design of the plane itself so the Germans would gain a lot of information from studying it.

A US Army air crewman tells Hogan he has just flown his 50th mission, and his tour of duty is therefore ended. US Army air crewmen were allowed to return home after flying fewer missions than that--25 to 35, depending on how late in the war their tour of duty occurred.

Since General Burkhalter referred to the P-51 as a "new" plane (meaning the P-51 Mk 1) the plane he and Klink take is not at all what they would have had available as it is a P-51D model. The Mk 1 was a razorback, not a bubble canopy model and had a 3 bladed propeller, not the 4 bladed one shown.

Moreover, a Messerschmitt engine could not be readily put into a P-51 Mk 1, as the Allison V-1710 or British Rolls Royce Merlin engines they used were based on English measurement connections not metric, which the Germans used.

Hogan agrees to steal one of the new Mustang fighters for the Germans; however, the model he steals is the later bubble canopy P51D. If the Mustang had just appeared, Hogan should be stealing the P51B razorback version.

When Hogan and Klink climb into the cockpit of the P-51, the upper side of the left wing is visible. The national insignia is missing.

During the debrief on the train, Hogan is in civilian clothes. When the debrief ends and he's allowed to go to sleep, he's in uniform.

Update: There is no problem here. It is night on the train, during the debrief. After the fade-out from the debriefing on the train, there is an establishing shot of the air base where they are staying while their statements are processed. There is no indication that this is actually immediately after the train debrief. Even if it were immediately after the debrief, there was also an order for them to be provided uniforms as soon as possible. The uniforms would have been issued upon their arrival at the airbase, and they would have changed out of their civilian clothes immediately, so they would not arouse suspicion while on the base.

When taking off in the P-51, it supposedly has a Messerschmitt engine. This would sound very different from the Allison engine that some P-51s had. The pilots would notice the difference. Nothing was said by either Hogan or Klink.

Update: First, Hogan would never have said anything about any difference in engine sound (it would have clued Klink in to his scheme), and second, Klink (even though he was a pilot in his earlier life) would not have known that the engine should sound any different from the sounds it was making, as he is German and has no experience with the P-51.

On the second night, Eva and Greta wear the exact same outfits and hair styles as they wore on the first night at Stalag 13. Therefore, it is safe to assume the 'second night' party was filmed at the same time as the 'first night' party.

The first decoration in the top row of Hogan's uniform is The Air Force Distinguished Service Medal - a decoration that was not created until after World War II.

When Col. Hogan and Klink are in their RAF headquarters private sleeping quarters, behind Hogan on the wall is a picture of a B-36 bomber with 6 propeller engines and 4 jet engines. This plane wasn't built until 1949.

After Burkhalter makes the offer to Hogan, he leaves and returns to the barracks. The gang turns out the lights and climbs into their bunks. When Hogan comes back out and gets them back up, the camera swings around the end of Baker's bunk and the shadow of the camera is visible on the bedposts.

When Hogan is setting things up with Klink, discussing the need to meet with Klink's contact, they are looking out the window at the planes. Klink puts his monocle in and says, 'Beautiful planes. No wonder we want one." During that line, there is a full reflection of a stage light in his monocle - not the window, as would be expected.

When Lieutenant Mills pours the two glasses of whiskey, he puts a small pill in one glass, which he holds in his right hand. He then turns and hands the glasses out, one to Hogan and one to Klink. The problem: he hands the doctored drink to Hogan (with his right hand), holding the bottle and the other glass up high, then takes the other glass from his left hand with his right hand, and hands that drink to Klink.

On the second night, Eva and Greta wear the exact same outfits and hair styles as they wore the first night. They would never have worn the same clothes on the second night as they did on the first night.

Correction: They would have, if they did not bring additional clothes with them. There's no indication when Burkhalter and the girls arrive that they had any luggage with them.

Update: After dinner the first night, Eva tells Hogan that they have a cozy little apartment (not a hotel room) in Hammelburg - so they would obviously have their entire wardrobe to choose from for the second night. Also, since General Burkhalter brought the women with him, he would have taken them back to their apartment on both nights.

Hogan was a bomber pilot and had been in Stalag 13 for quite sometime, so how did he know how to fly a new P-51.