You can see this bug on the Vaadin Reindeer Theme page:
http://demo.vaadin.com/ReindeerTheme
Using IE9, click on the “Tabs” tab, check “Closable tabs”, then click on Tab 1 thorugh Tab 6 under the “Minimal Style” variation. For us, sometimes we see nothing unusual until we click on Tab 5, but on occasion we see it jump down on Tab 2. If you reload the page, it seems to align fine again.
Also, if we p IE9 into “compatibility” mode, it seems to work normally, too, but not when it’s in standards mode.