I've been trying to go through a basic tutorial.
When I get to Step 7 (of creating the main two shapes), I need to use pathfinder to intersect between two shapes. But the intersect function's outcome is not exact (and actually other pathfinder functions.) I believe it's a bug.
This is how it looks with maximum zoom before the intersection:
And This is how it looks with maximum zoom after the intersection:
You can clearly see that the intersection wasn't made where the exact path of the yellow shape was, but a little bit downwards of that.
Here's two .ai files that correspond to both states for you to download: Before and After.