White points in the mapping background (Z Trassparency)?

[YQ-YSY]

I put a Plane behind the Cube, add PNG mapping, and choose Z Trassparency in the material.
After rendered, I found there are a list white point at the Cube outline (before the Plane only).
If I choose Raytrace in the material of Plane, without white points, all is OK.

I wonder Why?
And I don't know what situation I should use Z Trassparency or Raytrace?
[/img]