The Cumulative Security Update for Internet Explorer, which addresses numerous security flaws in Internet Explorer 6, introduces bugs involving the scrollbar. After the patch is applied, the page scrolls up or down twice when the user clicks once in the empty areas of the scrollbar. Clicking on the scrollbar arrows or dragging the scrollbar thumb works correctly.
From Binknu (read it because other little annoyances happen too in IE)
I also confirm having issues with the scroll bar. I discovered it when I was writing a post and lose the selection of the text everytime I tried to use the scrollbar.
Surely, this will be escalated by Microsoft support and fix soon. Thankfully this happen only with IE.