Tristán White wrote:

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.

IF you have a season ticket and go out of zones, and don't touch out at
the end of the journey you'll not be charged, it'll assume you're in
your zones. However, if you touch in out of zones but don't touch out
at the end, it'll charge you £1.00 for entering at zones 2-D, and
£1.50 if you entered in zone 1.