that technically if you buy a return ticket as the cheaper option and don't use the return sector, they can subsequently charge your credit card with the additional cost!
In reality I believe this will only happen if you flag up as a persistent abuser. There are many valid reasons why a return sector might be missed. On whom the burden of proof rests I don't know. It's happened to me a few times over a period of (many) years and I never had a problem as a consequence.