The interweb is full of stories of disgruntled American citizens that travelled to Europe/Schengen and were surprised to be turned back because their passport was not signed or not valid for x days.
The State department clearly says to its citizen planning to travel to Germany & Switzerland:
https://travel.state.gov/content/tra...itzerland.html
https://travel.state.gov/content/tra...s/Germany.html
Its amazing how much flexibility is expected of other countries
US agents have no issues sending people back on the same plane for far more minor issues and passengers have no recourse whatsoever.