What is the rarest chess notation?

There are many many notations in chess, but which one is the rarest?

Capturing adds an "x".

Checking adds a "+" at the end.

Checkmate gives the pound symbol at the end. "#"

What about en passant? Well, en passant is still normal like this: "exf5"

Have you seen a notation like THIS? You probably have, but do you know what it means?

Example:

"Nbe2"

Why is there a "b" before the "e"? 

Well, another knight may also have had a chance to go to that square, but if we say "Ne2",

we wouldn't know which knight it would be talking about. But if the "b" knight moved, it would say "Nbe2". Or if another knight traveled there instead, it would be "Nge2".

But... which is the rarest chess notation ever? Nobody knows yet...

Ngxe2++?