Dr. Felzer is introduced as working on Project 49. During the Manhattan Project, Plutonium 239 was referred to with the code of 49. 4 for the element number 94 and 9 for the atomic weight of 239. Also, Klink's new mailing address is Post Office 409.
There is a town called Freinsheim in Rheinland-Palatinate, Germany. This is in the general area where Stalag 13 is supposed to be located.
After being given the shot, Schultz starts counting down from 100 in English. He should have been counting in German. All of his other counting in the series is in German.
Per the title, for the entire run of the series Sgt. Schultz proclaims his ignorance by exclaiming, "I know NOTHING!"
In the scene in Klink's office when he and Burkhalter are meeting with Dr. Feltzer, it's obvious by the constant switching back and forth between Klink and Feltzer and Burkhalter that this was shot on different days. It's possible Askin was not available to film the entire scene on the same day.