Sure - but in that case the passenger could be said to have been careless. If he/she has no ticket because the machine wasn't working that is in no way the responsibility of the passenger. I'm assuming that the passenger is actually able to demonstrate that he/she did order the ticket (email etc).
Think we are getting into the same debate as the "How long should you wait at a ticket machine" topic i.e. there is no correct answer. In either situation one party is going to lose out - either the passenger who has to pay for another ticket but gets a refund later, or the TOC who loses revenue when people walk past the ticket machine without a valid ticket.