If you booted up Steam today to try to play a Valve game — like Dota 2 or Team Fortress 2 — you may have noticed something funky. Maybe Steam tried to boot up something different from what you picked. Heck, maybe you don't even own the game it tries to boot up at all.
One person had Dota 2 change to Left 4 Dead 2, for example.
Another person had Portal update on their computer...but they don't even own Portal.
Most of the incidents have something to do with Valve games fusing or swapping with each other, or people seemingly owning Valve games they've never purchased. It's weird. Naturally, though, not everyone is experiencing problems with Valve games right now. But if you are? Good news, chief. There's a fix.
I believe that refreshing your local configuration files for Steam might solve problems you're seeing.
Please click the following link and confirm the prompt within your client - that should refresh configuration of your client.
Link - clicky:
NOTE: If you have multiple installation folders for games completing this process will remove them.
Perform the following steps to add extra installation folders in your Steam client:
1. Open Steam
2. Click Steam > Settings (Preferences on Mac)
3. Navigate to the 'Downloads' tab
4. Click the 'Steam Library Folders' button
5. Click on 'Add Library Folder' and add the directory that contains your games
6. Click 'OK'
7. Restart Steam
Voila. That should take care of this strange issue. Happy Steam gaming!