You are designing a PCB.
On the PCmonitor, the design seemes to be faultless.
But when printing its masks (PNG files), the top copper mask or bottom copper mask, is printed partly fully in black. See example below.

The arrows show the locations where to look for a text, or a component surrounding,
is touching the board outline.  

I discovered that, at the vertical posistion, where the faulty printed black area begins (see the arrows) , a text comment, or the surrounding of a component, is just in contact with the board outline.

            The solution is :
Remove or relocate the text or component ouline a little, so it does not come in contact with the boar outline. Then re-print the design, and if OK seave the deign.