Originally Posted by Pharaoh

That is called back-to-back ticketing and while not illegal in the sense that you could be arrested, it is against airline rules. Penalties could range up to being stripped of your SkyMiles account and having to pay the full Y fare. Of course, only if you are caught, but many airlines have ramped up their enforcement.

There are a few situations where they will permit BTB ticketing on a special waiver basis but I doubt you fall into one of those categories.