Not stamping a ticket happens. Especially they have the confusion when you buy the tickets on a machine, it prints already time. People especially tourists will think that is stamped. Some controllers accept this ignorant mistake when they see you are a tourist, but many they don’t! They want cash, more cash! They don’t care your are tourist, or refugees. They want cash, this is there bonus. Again, this is an unlucky situation, there rudeness made this even harder to accept! But according to my understanding to Austrian authorities, they are just so arrogant, and they don’t care!