Getting charged the minimum fare is what I've experienced. However in
another post in this thread from Paul Weaver [1] he states that he got
charged £3.xx (he wasn't sure of the exact amount) when he only
touched out, which is why I wondered if the programming of the Oyster
readers might vary from station to station.

[1]
http://groups.google.co.uk/group/uk....cf502f0?hl=en&

Under normal cicumstances when you touch-in to the system the minimum
fare is deducted at that point and then when you touch-out any extra is charged.

If you only touch at one end of the journey and use a validator instead of
a gate the system will assume you are touching-in even if it is really the "end"
of your journey. It will deduct the minumum fare only expecting you to touch-out
somewhere else later.

If you do not touch-in but later touch-out through a gateline the system will
know you have missed touching-in. Perhaps in these situations the maximum
is charged. I'm not sure though.