In DistributeSpaceToTracks function (section 10.2), step 4, we have a loop that runs over all GridTracks. instead it should run only on TracksForGrowth.
Currently it results into inadvertent side effect that results into alternate span growth behavior.
Assigning all Grid bugs to Phil Cupp.
Fixed in Section 10.2 of the April 20, 2012 Editor's Draft: http://dev.w3.org/csswg/css3-grid-layout/