There seems to be an issue with sprites being drawn one pixel too high at certain times with platform movement:
<img src="http://i47.tinypic.com/2a4o67r.png" border="0" />
In the enclosed CapX you'll note in the layout the player object is clearly touching the ground, but will be one pixel higher when the cap is run. If you jump, the collision starts working correctly again. The bug also occasionally re-triggers when transitioning from the slopes back to the floor (Try moving back and forth a few times and it'll happen)
Thus far I've tried fiddling with Pixel Rounding on and off, but it occurs either way. Only workaround I can find at the moment is to make the collision box smaller, but this causes the sprite to appear to sink into the ground slightly when the bug is not present.