Can't say I've run into that issue, but I have had something else strange happen. Left 4 Dead 2 is being shared to me, but I don't have it installed. Suddenly, Left 4 Dead 2 Add-on Service has started appearing in the list of recent games when you click on the tray icon. A clean install of the Steam client cleared things up, but it came right back when I set up Sharing again. And it always comes back to the top of that list which is annoying. If I click on it, it prompts me to install but errors out with Invalid Install Path.