The only time I've heard that this has become an issue is not with an airline but the EuroTunnel channel tunnel rail link - there was a report on a recent consumer programme in the UK regarding customers getting a payment demand from the company because they had not used the return portion of a cheap excursion fare and therefore were charged for the more expensive one way ticket. I think it's made clear in the terms and conditions of EuroTunnel's bookings.
S.