We close all services with Aleinu, a prayer thanking God for making us different from idol-worshipping nations and hoping for a time when everyone recognises God.
In Aleinu, we say, "For they bow to vanity and emptiness and pray to a god who cannot give success". In the 14th century a baptised Jew spread the slander that this verse was a reference to Jesus. His "proof' was that "emptiness" and "Jesus" had the same numerical value in Hebrew. Never mind that the expression derives from Isaiah 45:20.
This smear made so much of an impression that Frederick the Great in the 18th century demanded the deletion of this verse. This censorship extended across Europe. By the late 19th century, some Ashkenazi rabbis re-inserted the verse. Today, it is found in many siddurim. Some people are proud to return to the original prayer, free of any fear of Christian persecution. Others omit it out of sensitivity to other religions.