Originally Posted by
BKKROP
The tax has been incorporated into the ticket for some time now, and by now, everyone assumes it has been paid to an airline, so it is most likely that nobody would be checking for that. If your airline made a mistake and did not charge you, I don't believe it is something that you should worry about....unduely, or even at all

I was stopped just a few months ago when checking in with LX in F. AC Aeroplan had not charged the tax. I was told I needed to pay the 700THB tax for each ticket...cash only.
On other trips the check-in agents have missed the fact that the tax had not been collected by Aeroplan.