Was just in Prague. Loved it. Only one bummer the whole trip. Went with my wife and two kids up to the palace and took the tram. Bought tickets, climbed on and rode a street tram. As we approached, a guy flipped out a "transportation cop" badge and asked to see my tickets and another ladies ticket. She was a local and had a pass, I handed him my tickets.
Apparently there is some very small print in english only so they catch english speakers, that you are supposed to validate the tickets. How do you do this ? It doesn't say. It turns out there are these very small little boxes (with no writing no them) that you have to insert your ticket to get a time stamp. We had no idea. I explained we bought the tickets 10 minutes before and clearly we weren't trying to rip off the system. All he would say is "these tickets aren't valid. You read english it says you have to validate them." I said, show me where to validate them ?? He showed me the box with no writing. We had seen no one else do it so we didn't know.
Long and short of it, 700 Czech pounds later we were off and running. It is clearly designed to zing american and western european travelers. We got off the tram and heard some other people arguing, a group of six american asians who had done the same thing. It's pay or they call the cops. And the only writing on the ticket is in english, they don't try to get anyone else.
So, lesson learned for 40 dollars american. And a good way for the Czech's to turn off honest tourists. We did buy tickets, we weren't mooching. So Beware... if you ride the trams / buses, buy the tickets and get the stamp from the unmarked box or you too will get nailed. I saw dozens of westerners getting it after that. It's a well planned out market for them.