This is the first div.
This is the second div.
In this minimal test case positioned float with z-index:-2 has a red background and a negative top margin. This causes a positioned div with z-index:-1, with a green background and earier in the source, to pull down on top of the second.
The div with the green background has higher z-index and so should be painted on top of the red positioned float. However, in IE 8 beta 1 the red positioned float is painted on top of the green div.
View implementation bugs related to z-index and stacking contexts.
For a discussion of the ambiguities and errors currently present in the CSS 2.1 Candidate Recommendation as regards z-index and painting order, see CSS 2.1 Candidate Recommendation : z-axis layering behaviour, stacking contexts and the z-index property.