On Mon, 18 May 2009 11:00:51 +0100, Walter Briscoe wrote:

OUT IN Station
Y - KXSP NR
- Y KXSP LU
N - Farringdon
N - Barbican
Y Y Moorgate
N N Liverpool Street

As it happens, my investigations were backed up by a Z1-3 Travelcard and
the ticket stopped working at Moorgate after it had been rejected at
Liverpool Street; it worked at Moorgate after rejection at Farringdon.

It probably stopped working at Moorgate after it had been accepted at
Moorgate.