Fix for "Failed to start game (app already running)" error - Steam

I got a fancy new 144 hz monitor the other day and was messing with some settings on Borderlands 2 to push it up past 60 fps but after poking around a bit, I couldn't load the game and instead got this error:

Failed to start game (app already running)...

I looked online and found a number of potential fixes, none of which worked for me, including the ill-advised "run Steam as an administrator." -_-

Thankfully, someone mentioned that they had also been using the graphics enhancement program SweetFX before getting the error. Removing all of the SweetFX files cleared it up for me, as well.

Here is a list of all of the files used by SweetFX (found in the Borderlands 2 Binaries\Win32 directory, in my case):

d3d9.dll
d3d9.fx
dxgi.dll
dxgi.fx
injector.ini
SweetFX readme.txt
SweetFX_preset.txt
SweetFX_settings.txt
and the SweetFX folder

Move/delete these files/folders from your executable's folder and you should be set.