Berlin sucks. A bit confusing and then they pay guys to walk around and check for tickets.
That's my experience, at least.
When I was young and poor I "saved money" by buying the ticket in the morning, and only stamping it on the way back in the evening. I gambled successfully that if you get caught in the morning then you are the foreigner who does have a ticket (time of purchase is on the ticket) but "was not aware" that you had to stamp it.
And yea the "clueless tourist" approach works almost always.