If draw a polygon with depth value, then read it back, there is one pixel bias.
Please see bug #15194 comment6.
The bug exists on both i915 and i965.
The bug results from the PolygonMode is set as GL_LINE.
If set the PolygonMode as GL_LINE, then polygon will be drawn with one pixel bias(only the left line).
Created attachment 15888 [details]
The spec allows the line rasterization to be off by a pixel in X or Y. But in the case of this test, the lines are being drawn exactly on the pixel boundaries, so if the test was fixed to draw the lines along the middle of the pixels, it would work fine on our hardware (and probably everyone else's, too).