A few weeks ago I noticed something strange about the ticket machines of Deutsche Bahn. Normally, if you miss to hit a button, a red dot on the touch screen indicates where your "click" was recognized. But when viewing the Verkehrsverbund* section and "missing" on certain spots on the upper left and upper right corner of the screen, you saw an hourglass cursor for a short moment.
Being in curiosity mode I had to test if I could provoke a feedback of some kind. And in fact, rarely the screen was like flashing as I was rapidly clicking on both spots simultaneously.
After playing around a bit more I found out that there are two "four click combinations" that will bring up a screen with some information: left, left, right, right gives you the application and system version; left, left, right, left a hole bunch more version data, the location of the machine and whatnot.
At the point where I had found the first working combination and began trying out others I kind of felt like in a good old point and click adventure. :D Thanks for the entertainment, Deutsche Bahn. ^^
*An association of public transport authorities.