About one year ago I was sitting with another 3 people on FRA-STR (very short flight) in C-class. The doors were already closed when a LH FA came and asked to show the boarding passes. One person had to move to Y-class afterwards ...

I expect that the staff does check if the passengers are sitting in the correct class (airline, train). If the staff close their eyes, will they do so also on other occasions (indifferent service?, security?)!?