In a container (the light gray box with striped background and border) there is a box with a margin-top. In IE7-/Win this is ignored if the box is preceded by an absolutely positioned sibling. In IE7 this happens also when the box is preceded by a fixed positioned one.
If the box precedes the absolutely positioned one the problem does not occur.
The presence of an empty box between the absolute and the static one fixes the problem
position:relative on the container (and/or on the inner box with the margin-top) has no effect on the problem.
In the above cases both the container and the static box have hasLayout. Presence/absence of hasLayout causes many similar problems with nested boxes: IE7- wrong collapsing, disappearing of margins and hasLayout. The current problem does not occur if the static box does not have hasLayout, but of course this is a rarely usable fix.