The first foux pas

After arriving very tired in Prague, I attempted to buy the right tickets to get myself, cheaply, from the airport to the hostel. I managed to buy the right ones, but failed in realizing that the tickets had to be stamped before they were considered valid...
Needless to say, I looked like a foreigner with my bright orange backpack, sleeping bag and purse. So, I was the one that the inspectors pulled over to the side to ask for my tickets - which I proudly produced.
I will never forget to validate my tickets again. But what interested me, was that the man telling me the fee was angry with me, and defending himself from the beginning of the conversation. It was as if he expected me to argue and call over his boss. It makes me wonder what sort of behavior Americans usually exhibit.