I recently returned to Thailand from Europe, where my Thai Airways ticket had me only entering Thailand (with no return Thai Airways ticket) outside of Europe. I had read one really should have a throw-away ticket, so in fact I did buy one, which I posted about previous on this forum ( I bought a legitimate Krabi to Kuala Lumpur one way ticket for 740 Thai baht.)
In Frankfurt, Thai-Airways did not ask for my return ticket. BUT I suspect they saw my old Type-OA visa (in my passport), so my old permission to stay, and saw (an no longer valid re-entry permit) and mistakenly thought I had an appropriate re-entry permit, which I did not have.
In Bangkok, the Thai Immigration Officer (IO) asked for my ticket. I passed him my boarding pass, and I was told, "NO - ticket". I did not now which ticket he wanted? The ticket to fly to Thailand, or my ticket showing me leaving Thailand. I was rummaging through my backpack to get both tickets, during which the IO ws talking to my wife (in Thai).
I have no idea what was said, but then the Thai IO waved me through, and I did not have to show a ticket. I asked my wife a week later what was the discussion she had with the IO, and she could not remember it.
...
In the past few days, a friend of mine entered Thailand from Vietnam with a one way ticket (Vietnam Airlines). He was ask to show a departure from Thailand ticket. He was entering Thailand Visa Exempt (as his previous Type-OA visa permission to stay had expired). He had a throw-away ticket (legitimate) that he showed them, and they allowed him to board.