Have you read about expats being asked for a return or onward ticket below?
All immigration agents have the discretion to ask for evidence of departing Thailand.
If a traveler reads your experience and decides to enter Thailand without evidence of departing, and they are asked for evidence of departing by an immigration agent, their excuse that ‘Mathew said I don't need departing evidence’
.. will not be accepted.
BTW.. I have talked to several other normal looking tourists who have been asked to show evidence of departing Thailand.
as side bar to the above:
Once the ETA is implemented, everybody will be required to list evidence of a return or onward ticket.