This test case demonstrates the quirky behaviour of IE8 RTW where there is intermittent margin collapse of sibling element upon selecting text within preceding same-type sibling within same list-item.
Since this behaviour is intermittent, its very hard to diagnose exactly how or why it is occuring.
Select and highlight the first five list-items below. Whilst still retaining that selection, very slowly select the list-item, 'AND NOW THIS...'. Although intermittent, you should be able to observe the margin collapsing (or at least a noticeable jump), between this last list-item and the latter portion of text highlighted in yellow.
From my analysis, I've included that it's possible this bug can only be reproduced upon using a list (OL or UL) when the two DIV siblings (highlighted in yellow) are nested in one list-item.