Fullscreen sometimes gets stuck after Quit on OS X
I do not know if this happens on Linux or Windows too, but on my machine at least it does.
If I set the game to run Fullscreen in the Config file, once in a while, no predicting when, after I quit the game the game's last frame gets stuck on my screen with no way to kill it or get back to the rest of the system. According to the system, Love is no longer running at this time so it can't be killed. It's like the WindowServer forgot to get rid of it or something. I don't know what exactly causes the echo.
After quitting, Löve will fade down like normal, but then flash back to whatever was on the screen at the time of quitting.
The only way to get out of it is to force the computer to shut down. I usually end up going to my other computer and SSHing into the laptop to "sudo shutdown" since "killall love" returns "No processes". Even while the shutdown sequence is in process the image stays on screen covering up what would normally show up while OS X is busy shutting down which is a spinning icon over the desktop picture. So it's definitely something to do with OpenGL or the WindowServer or something. I don't know how the insides of Löve work so I can't be sure. I hope this can help though.