White pixel line around shapes
Ever since Appearance was introduced, my artwork on screen suffers from a white pixel line around shapes. Seems a rendering problem: the line is not actually existing - zooming in does not make it thicker, it's always one pixel. Tried switching GPU off and on. No difference.
The appearance of the black shape: Fill is black, Stroke is None.
I don't know if this is the problem you have, but there's a bug/flaw with the anti alias in Illustrator. Even if two objects are lined up perfectly and there's no gap between them there will be a thin white line. If you turn off "Anti-aliased artwork" in the preferences (and turn off GPU acceleration) the white line disappears. But you get an ugly preview, of course.
The only way to avoid it is to always make sure objects are overlapping each other.
Jan Vork commented
Update: the artefact only exists in CMYK mode, when black (K) is involved. Adding the adjacent color to the black makes the white edge disappear* (but setting the black shape to Overprint Fill doesn't...). Feels like the solution you need when designing for offset printing, to make sure colors register nice, whitout white gaps.
Seems like Illustrator is not equipped to handle anti-aliasing between CMYK colors correctly.
*) e.g. a black circle (CMYK 0/0/0/100) on a cyan background (100/0/0/0) shows the white edge. But a black+cyan circle (100/0/0/100) on a cyan background doesn't show it.