[chiph]

Could it be because you're a foreigner? I almost certainly got picked out on every ride on the Munich S-Bahn by a fare agent for a ticket-check because I was obviously not German.

Which I totally get - a tourist is way more likely to make a mistake than a local.

[Zak]

It has been my impression, having spent a significant amount of time in Germany, that German ticket checkers aren't primarily trying to detect people who have made a mistake, but those who intentionally ride without paying.

[cobbzilla]

This information may be a bit dated, but I visited Berlin circa 2008 and my friend (American expat) who had been living there for 10+ years insisted that we not pay at the subway, because no one checked. Following his guidance, I never paid for the subway, and was never fined. This was just within the city of Berlin, though.

[tormeh]

This varies in place and time. I think close to 50% of my travels on a recent trip were checked.