I had a paper ticket, remember the guy took them, he couldnt have taken them if they were on my phone - or maybe he wouldve snatched the whole thing who knows! oh my tickets never works at waterloo because they always have the locks on the gates so have to manually check most.Were you using a paper ticket or an eticket? And no error codes like "return already used" or "not valid at this time"? That seems like a big issue with the gates at Waterloo to be honest if they are permitting travel with what is supposed to be an invalid ticket.
and to correct you, it is a valid ticket. I would have thought e-tickets with the return scanned first WOULD work, but would not if the outward journey part had already been scanned, not sure though someone would have to test it.