Before the wing snaps on the model glider, a quick shot shows the plane to have wing struts. The model glider had none.
If the wingtip of the glider was visible from the castle courtyard, the launching track should have been visible as well.
In the opening scene of the film, Cook sees his contact and a boat on the opposite side of the river. Cook then meets the contact without ever crossing the river.
When Cook and Crawford fight, Cook has a cut on his right cheek before Crawford ever hits him in the face.
The cut on Cook's face is completely healed by the following morning.
Fitz makes reference to a balsa wood strut on the model glider. The prisoners would not have access to tropical woods.
The only POW camps remotely near Switzerland were Rottweil and Memmingen. No POW camp would have been built within visual sight of neutral Switzerland.
Allied prisoners would not be in possession of dress uniforms.
Allied prisoners would not have knowledge of songs that the Jews were singing in the concentration camps.
A Nazi book would not be titled in English.
A balloon can be seen holding Flaherty's kite aloft.