that it could charge you an extension on the basis that you might
have gone
all the way to Cockfosters and the barriers were open (i.e. it would
leave

If you touch in within your travelcard zones but don't touch out it
assumes you are honest and charges nowt. If you touch in outside your
travelcard zones and don't touch out it assumes you aren't.

I beleive this has been discussed before - have a look at the archives.

I think the conclusion at the time was that it assumed you were honest, for
now, but in the future it would take from your pre-pay balance.

There may be other reasons to touch in and out, like the system may identify
you as having purchased two short travelcards, changing at an ungated
location. I imagine it could flag this as unusual behaviour assuming you
have bought two travelcards for validities at each end of your travel, and
nothing in the middle.

Cheers,
Dave