Soap made from human corpses
The "human soap" stories may have started because with the bars of soap made by the government were marked with the initials "RIF."  Some people thought this stood for Reichs-Juden-Fett in German.  This means "State Jewish Fat" in English.  (In German acronyms, "I" and "J" were often used like the same letter, so people thought "RIF" could mean "RJF".)