The blue square box on the left is set to float to the left, it's followed by a green rectangle set to an absolute position with no coordinates specified. Both boxes are separate from eachother in the HTML.
Mozilla and Opera both position the absolute-positioned box as if the floated box doesn't exist, which is personally what I'd expect, however Internet Explorer decides that the floated box should have some say in the positioning of the absolute-positioned box. I didn't find anything in the W3C recommendations that described what should happen here, let me know if you find something that clears this up, and I'll mention it here.
This IE bug was causing a problem on my main site, in that circumstance using 'clear: both' on the absolute-positioned box fixed my particular problem, however the box still won't place itself in the upper-most left corner of its containing element (body) by default.
If there's any way to make IE behave correctly let me know.