On Tue, 17 Oct 2006 22:39 +0100 (BST), Colin Rosenstiel wrote:

Nor (again AFAIA) will it cause any problems if you touch-in a
second time, say if you weren't sure if you're first attempt was
successful.

The problem is when you touch in when you should have been using a paper
ticket for the particular journey. There appears to be no way to undo
that mistake at the time. It's easily done if you have to use a
combination of tickets.

Yes, this can't be all that uncommon. Only a few weeks ago I was
travelling with someone who accidentally used his PAYG Oyster to enter
the barriers at a Tube station, when we were travelling on ODTCs (as
we were using NR that day too). I sent him to the ticket office to try
and have the mistake undone. He returned an impressively short time
laster with it all sorted.

I can't understand why they'd want to make it so much harder for
people. If anything, they should be making it easier (by allowing
staff to do it even outside the ticket office opening hours).