Why am I seeing in 3d?

I just installed the Stanley parable as a mod, and upon first launching the game, I've got the red-blue tinges on the corner, like the game is enabled for the red-blue 3d glasses. I didn't enable this, is there some way I can disable this feature, maybe a Source engine solution?

I didn't realize this was a factor, but I did just install Windows 8.1.

I had an identical issue when I upgraded Windows 8 to Windows 8.1 - as part of the installation process the drivers for my nVidia graphics card were reinstalled, which included the stereoscopic 3D drivers and defaulted them to enabled. In my instance I got the 3D effect in Payday 2 rather than the game you're trying to play.

I resolved this issue by uninstalling the nVidia stereoscopic 3D drivers through add/remove programs. If you're actually ever going to use stereoscopic 3D you can disable it by unchecking the "Enable stereoscopic 3D" checkbox under "Stereoscopic 3D" in the nVidia control panel:

I downloaded the latest Nvidia driver for my GTX 680 and that solved the problem - also added the stereoscopic option that was not previously there. The game it was happening on was Mass Effect.