The PGN "Standard" (if you want to call it that, because it is very flawed and seems out of date), states:

> 
> * First, if the moving pieces can be distinguished by their originating files, the originating file letter of the moving piece is
> inserted immediately after the moving piece letter.
> * Second (when the first step fails), if the moving pieces can be distinguished by their originating ranks, the originating rank digit
> of the moving piece is inserted immediately after the moving piece
> letter.
> * Third (when both the first and the second steps fail), the two character square coordinate of the originating square of the moving
> piece is inserted immediately after the moving piece letter.
> 
> 
> 

What I want to know is this: what situation could give rise to the third case?

As I think this through, I cannot for the life of me think up a scenario that the first or second disambiguation rules wouldn't comprehensively cover.