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"O sadness, O heartache! Is this not to be lamented?" is the English translation of the Choral BWV 404: "O Traurigkeit, o Herzeleid! Ist das nicht zu beklagen?", based on a hymn by Johann Rist written in 1641. If you do not know, 404 is the HTTP result code when you ask a website for a page it cannot find... Bach again could not be more appropriate.
Johann Rist's hymns also served as the inspiration for no less than three cantatas:
So Bach has all the ingredients to make a boring thing like a "page not found" result more exciting!