On a couple of occasions, I have been accidentally booked on 'cabin baggage only' and not realised until turning up with trolley case and my usual laptop bag (Usually, if on cabin only, I have a small bag that fits inside my trolley case).
Solution - buy something in the shopping centre that is now the departure lounge and ask for the biggest bag they have (Harrods ones are good!). Laptop bag then goes in the carrier!