1 000 000 000 Mark Deutsche Reichsbahn

Deutsche Reichsbahn issued its own emergency currency during the hyperinflation of 1923 because the Reichsbank simply could not print fast enough to meet demand. State railways, municipalities, industrial firms, and even department stores all became de facto note-issuing authorities that year — not by legal right but by practical necessity. The Reichsbahn's billion-mark notes were notgeld in everything but name, accepted by workers and merchants because there was nothing else to hand.

The watermark is the sole security measure, which tells you everything about how briefly these were expected to remain valid. By November 1923, a billion marks wouldn't cover a tram fare.