I had my letter stating the details of the 1952 bi-lateral trade agreement between Germany and Australia allowing Australians to stay an additional 90 days in Germany. At the customs / police checkpoint there were some questions asked, a VERY careful reading of the letter, more questions, another read of the letter, and my passport was stamped!
I arrived in Germany on the 29th January, leaving for Holland about the 14th February, and spent less than 90 days in the rest of the Schengen so I was quite legal. My concern was that the authorities might not think so, and my policeman was heading that way till I produced my letter! As I said, I haven't left the country yet, but I'm over the worst.
Relief at the airport!