I know there have been some legal issues in Germany about this, but I was surprised to see the following fare condition on a Z-class fare ex-UK to HKG:

Change to OW travel: If a totally unused return ticket shall be used for one way travel - either inbound or outbound - the difference between the return and the applicable one way fare has to be collected.

Has anyone had any experience of this? How enforceable is it? Surely anyone wanting to travel OW only needs to book a return 12 months later - I can hardly see even LH knocking at your door a year later to try and collect a fare difference.