While it is unlikely you would ever be charged criminally, it is certainly possible. AC*SE once posted an explanation as to the hows and whys.

But even if you aren't charged, you might find one day that AC wants you to pony up for the difference between the return web saver fare and the normal one way fare (read full Y fare) before they let you travel any more. Alternatively, you might get the Robert Laurie treatment, and have your FF account cancelled and your points revoked. You might even be banned from travelling on AC. There have been a number of each of these types of responses in the past.