I'm not sure what the real rules are, but given that ticket checks are not done when entering a train, but by the conductor walking through the train, in reality nobody can know if you really skipped the train or not. It might just have been, that the conductor didn't reach your seat to check the ticket.
At least in Germany with Deutsche Bahn there isn't even a rule, that you have to use the ticket as booked, and i would assume that ÖBB has similar rules as DB.