The Missing Niggun
There is a missing niggun from Sefer Haniggum, Niggun 299. It skips from song number 298 to 300, with no space in between.
By looking at the index, It seems like the missing song is called Niggun Hisva'adus.
There is no way to find out which Niggun Hisva'adus this is. Because there's millions of them.