Apparently, from 25th September the charge for an incomplete Oyster
pre-pay journey starting or ending at the NR termini at Marylebone,
Euston, Liverpool Street and Fenchurch Street will be increased from
£1.70 to £5.00.

Furthermore, LU ticket offices will be unable to remove the £5 penalty
- if you receive one in error, you will need to phone the Oyster
helpline. Presumably this means that no reimbursement will be
available to holders of unregistered Oyster cards.

So if you're using pre-pay on NR to or from the above stations, be
extra careful, especially if your card is unregistered. Make sure the
person in front of you has passed through the gates and the gates have
closed before touching your card on the reader. If you have trouble
getting through the gates and a staff member at the side gate asks you
to touch your card on the reader there, I would refuse unless you're
sure the £5 charge would not be applied. If you're travelling to/from
an ungated station, don't forget to touch in/out - remember it's
easier to forget if you're travelling in a group.