That's an amusingly mathy way to put it.

"There is a technical error concerning the bibliography: A citation of the form [n] with 52 ≤ n ≤ 121 refers actually to item n – 1 of the references (there is no citation [51])."

So how many references are there in the bibliography?

(a) 120
(b) 121
(c) 122
(d) This kind of thinking doesn't interest me.