If the browser width is less than about 900px, in IE7 an unnecessary horizontal scrollbar shows up. It seems caused by the presence of content going outside an hasLayout and shrink-to-fit container which is somewhat positioned to the right.
In this test case the content is a blue div with margin-left:-1px, which pulls it outside its container. The container is a widthless right float (green top border). This is contained in a red top bordered box.
The same problem occurs with other shrink-to-fit containers, e.g. when the blue div is a widthless absolutely positioned box. Even when it is an inline hasLayout box, with text-align:right on the parent, and the content is italic text (which goes "ouside" its container). See also this similar test case IE7: unnecessary horizontal scrollbar caused by italics.