I am using the latest Steam client available on Ubuntu Trusty.

Take Terraria for example. Terraria for Linux is out for public beta, but otherwise is Windows-only. Terraria appears in my Linux/SteamOS games list, and can be installed from any list in which it appears. The install will schedule and be added to my download manager, and will immediately succeed having downloaded zero bytes, unless I opt in to the beta program (which I have since done). The zero byte install will then have a "Play" button, which will register that I have played the game today, but fail to launch anything because absolutely nothing has been downloaded.

Best thing to do would be to display that the game is available as a beta. Second best thing to do is error out because only beta versions are presently available. The worst thing to do, in my opinion, would be to fix this by not displaying it as available, because the fact that Terraria was listed as something I could install is the only reason I stumbled upon the beta program at all.