I've spend most of today doing various tests with the BMPCC.
I was able to create this pattern but only in some circumstances. So I tried a few other things and I think Frank has a point.
I'll try explain. I used to get issues with the FS-100. If I had light in the frame it would cause internal reflections where the light would reflect off the sensor onto the rear element and then back onto the sensor. So you start getting ghosting images are strange flares. I think this is being caused in a similar way.
So this is how my test ran. Camera set to 1600ISO, aperture around f/5.6. Dark room with charcoal curtains as my back drop. My light source was a mini maglite touch.
I put an old Nikon 55mm f/1.2 lens on the BMPCC with the Metabones SpeedBooster. Shine the light across the lens and create flares. Catch the light direct in frame. Shane the light on the curtains. When I review the footage I get noisy footage as expected but I'm not seeing the grid that others have posted.
So I remove the Speedbooster and use a normal adaptor. Same process. When I review the footage most of it is normal noise. But when the torch shines across the lens a pink flare is produced and the pixel grid appears.
So I then take the lens off and create light leaks with my hand. No grid.
I need to do more testing by I think, as Frank mentioned, that it might be to dow with internal reflections between the sensor and the rear element of the lens. Looking at the other shots that have this issue it seems that it happens where there is light flare.
Screen shots here:
https://www.dropbox.com/s/bkt91flu1l3np ... test_1.zip