Referring to the Rainbowduino with official seeedstudio firmware (as well as the 3.0h firmware).
In the green channel only, the LED directly below (row y + 1) the one being lit is always on faintly. In a dark room, you can see this effect more. You should be able to replicate this by loading a slightly modified firmware that fills the front buffer with nothing but one dot at (0,0) with RGB = 0F0. The LED at (0,1) should be glowing faintly.
Is this a hardware issue? Or … does it happen in all channels but our eyes are just more sensitive to the green in particular?
I modified the firmware to do nothing but light first two columns (to reduce the PWM-related issues to minimum) and I’m still seeing this faint glow.