I have pixel grid turned off. I have all snapping turned off. But yet, when I try to combine two objects with an even size using pathfinder it adds .083mm to it. Any idea how to stop this? See images before and after:
Illustrator's Pathfinder is only so good, meaning, it isn't 100% precise. In your case I believe you may have hit one of those very easily noticeable cases. It may also be due to the "Align to Pixel Grid" being on.
As @joojaa mentioned:
You have align new objects to pixel grid on. This causes the merge to snap the points to the pixel grid. You can see this in the second image
You can go to Window → Pathfinder → drop-down menu → Pathfinder options
Now change the precision. The lowest (and default) you are able to set the precision to, is
0.028*. (That's in CS6, which I use, I'm not sure about CC).
*If you write
0.000000001 it automatically changes to
Some people talk about it on Adobe Forums here: https://forums.adobe.com/thread/2246376
I'm not sure if this is the reason that you're having issues since nobody was able to replicate it, however, it does seem so, based on the Adobe Forum discussion.