See, e.g.: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=css2.1%2F20110323%2Fabspos-containing-block-initial-004b.htm%2Ccss2.1%2F20110323%2Fabspos-containing-block-initial-004d.htm
Fixed by http://trac.webkit.org/changeset/133399.