Fairly common, it used to be more common - remember the days of the Consolidator fares? If this was a cheap ticket, then there is an element of you get what you pay for. I think were forums like this become useful is providing online information so that travellers can be made aware of these limitations, prior to the internet, travellers would have no clue what was going on behind the scenes.

You are effectively in a seat sold in bulk at a special price for cruise line companies (etc), and some travel agents get hold of unused allocations for general sale. They are highly restricted, low cost and often not open to TPs or Avios. Caveat emptor, methinks.