Artifacts when using opacity mask

Question

I get artifacts or outlines of the objects I want to mask when using opacity masks.  They can be seen when in AI format but saving to png they are not visible.
Example:
If I mask a colored rectangle to fit within a circle I see the outline of the rectangle outside of the cirle.  It does get maksed within the circle it just leaves those artifacts outside.
Help?

Answer 1

I get artifacts (very thin red line on bottom of eyeball shape in stat fat) when I put for example 100% black Multiplied over 100 pms red.  They are in perfect register, and the the thin line does not appear in the final printed piece.
Your situation may be different, but  if the artifact does not come up in a png, then you should be safe. You can also view single color channels by using window >> separations preview >> check overprint >> turn off eyeballs for all but one channel, to proof your work.