I've experienced som kind of bug when displaying my fixed layout epubs on older Apple devices. It happens on iPad 1 and iPhone 4 (and presumably on 3GS), but not on iPhone 4S and newer devices.
There seems to be a thin grey line showing up next to images at some zoom levels, probably at the edge of the placed image. This seems to have nothing to do with any background or how the image is cut. It is not visible on all images, and as stated, on most devices it does not show up.
I have no idea what is causing this, and web searches cannot answer it either. But is certainly does not look good.
The screenshots are from an iPhone 4, but it is even more apparent and disturbing on iPad 1.