Funny Thing Happened on the Way to London - S3-E5
Plot hole: When the driver of the car addresses Roberts (the real Roberts that is, not the double), Roberts reprimands him, telling him not to call him Baumann. It is safe to assume the guard was supposed to be speaking German, since he addressed Roberts by his German identity. That poses some interesting questions. Did Roberts speak English? In those days, most Germans did not even have a basic knowledge of the English language. Did he speak German? That would imply he has a good enough command of the German language to pass as a German. Roberts however isn't a spy like Hogan and his crew, but a pilot in the RAF.
Funny Thing Happened on the Way to London - S3-E5
Other mistake: When Lieutenant Bauman starts speaking German, his last two words "especial mission" is said in a decidedly French accent. The correct German phrase would be "Sonderauftrag." Non-German terms like "Mission" were generally avoided in the Wehrmacht for ideological reasons.
Funny Thing Happened on the Way to London - S3-E5
Revealing mistake: When the staff car enters the camp, you see a set of tire tracks leading up to, and stopping at, Klink's position. When the car comes in, it precisely follows the tracks, suggesting previous takes.
Answer: The core POWs regularly escaped the prison camp but they made it their mission to conduct espionage and commit sabotage in the surrounding German territory. They also collaborated with different underground resistance groups and used a network of secret tunnels to help prisoners from other POW camps to escape, who then relayed vital information back to the Allied forces. Hogan and his men maintained the illusion that Stalag 13 had never had any prisoners escape in order to avoid their covert operations being shut down. Being that the prison camp is set in Germany during WWII, there were no American embassies.
raywest
Hogan has mentioned to different characters that they are actually stationed at Stalag 13 to help allied soldiers and prisoners from other Stalags to escape Germany.