As a follow-on to the posts discussing (theoretically) who pays to repatriate a passenger denied entry because of incorrect documentation:

We were booked from BKK to SGN. We have NZ passports and, at that time, we had to have a visa to enter Vietnam.

We got our visas about 3 weeks before departure and checked in at BKK. The agent at the Thai Airways counter checked our visa at check-in.

When we arrived in SGN, we were denied entry to Vietnam and, after much shouting by Vietnamese officials, at us and at the TG staff, Thai Airways was required to fly us back to BKK.

When we arrived back in BKK, we were informed that, due to a Pan-Asian conference in Hanoi the weekend we traveled, Vietnam had declared our class of visa invalid. Previously, they had been phasing out our class of visa and phasing in the new class but, due to this conference, they had arbitrarily declared our class of visa to be invalid.

Apparently, Thai Airways had been notified only the day before our travel that they should not let anyone with our class of visa onto their flights to Vietnam, but this information had apparently not filtered down to their check-in agents.

We found Thai Airways' HQ in Bangkok and were issued with new tickets, BKK-SGN and SGN-BKK, at no cost to ourselves.

I do not know if Thai were fined, but I do know that they had to bear the cost of our additional flights and that they made no attempt to recover anything from us.