I can't actually see the Watford Junction cap listed anywhere.

http://www.tfl.gov.uk/tickets/faresa...ares/5196.aspx
http://www.tfl.gov.uk/tickets/faresa...ares/6769.aspx

In any case caps can overlap, so the z1-8+w cap would apply to all
journeys except those to Amersham or Chesham, and the Z1-9 to all
journeys except to Watford Junction.

Yes. But if someone (for whatever reason) travels to Amersham AND WJ? I
don't know if I ever do this, but there has to bee a cap and I hope it's
below the sum of Z1-9 and Z2-8+W (or Z2-9 + Z1-8+W) :-)