Originally Posted by
dranz
It may indeed get you a bit of extra scrutiny the next time you fly, but if you've got green cards, it will not be a criminal matter.
Irrespective of green card, no green card, entering the US on ESTA, or even as a US citizen, not using the return half of a round-trip ticket would
never be a criminal matter. It might put the OP in breach of the airline's ticket rules but that then potentially becomes a civil matter between them and the airline.