If someone has a season ticket on their Oyster, plus top-up on their card
for whenever they need to go out of zone, and they don't touch out at the
end of their journey (eg travelling to Maryland and forgetting to touch
their card when changing trains from the Central line because they're in a
hurry).... what will happen?

Will £4 come out of their top-up? Will they have a problem at the gates
next time they go through? Just curious.