Walter is killed when a spark from his cigarette lights the leaking railroad tanker car full of phosgene gas, and it explodes. In fact, phosgene gas is actually non-flammable.
The gas in the train is told to be phosgene, but phosgene is not flammable, so it could not cause the explosion.
The battle scene where Gereon abandons Anno purportedly takes place (according to the subtitles) on November 14, 1918. Yet, the Great War ended on November 11, 1918. The German voiceover (by Anno) states the date as November 4th 1918.
Gereon Rath performs CPR on a drowning victim. This technique was developed in the 1950s and wasn't widely adopted until the 1960s and 1970s.
Svetlana Sorokina performs "Gloomy Sunday" in Russian in Paris in 1929. The song was written in 1933 by László Jávor (words) and Rezso Seress (music).
At Dr. Anno Schmidt's desk there are two modern versions of the Bestlite BL1 desk lamps. Also the original BL1, which has a slightly more curved neck, was designed in 1930.