I was coding a kind of generator for the emulator, so that its GLSL shaders get automatically generated/compiled whenever some significant parameter is dynamically changed (such as the window size, or the filter kernel size and/or values).
The generated shaders obviously had a bug in the coordinates computation (that made the texture lookup fetch texels too far along the X axis).
This bug alone had nothing uncommon... until for some reason I forgot to clear the content of the back-buffer before each rendering loop. This then gave this strange feedback effect that I'd like to share :
It's even more amusing when you see it moving in real-time. But I guess you'll never see it alive, since I'm about to eradicate those nasty bugs... ;o)