Will I be denied boarding in Europe or immigration in Thailand with a one-way ticket and onward travel to Cambodia or Laos?

One way ticket. so i have this question, if you buy a one way ticket from let's say Europe to Thailand, BUT you have an onward ticket to cambodia or laos within 30 days, is there a chance they might deny you at airport europe or immigration Thailand ?

because after all, the plane ticket to the neighbour country is a proof you can (and should do), leave the country.

(of course it's no guarantee you will effectively use this ticket, but nor is there a guarantee you use the return ticket if you buy a return ticket to europe)