An OWA user I support complained that she could not use her scroll bar to advance through her inbox after upgrading to Exchange 2010. She keeps her web browser scrolled to 150% to mitigate eye strain. She can advance through her messages one at a time using up and down arrows, but she's accustomed to clicking the down-pointing triangle at the bottom of the scroll bar to advance through the list.
I confirmed on my computer that the scrolling doesn't work when IE is zoomed to 150%. I also found that Firefox starts stuttering, then fails to scroll, when zoomed to the second degree (ctrl++).
EDIT: She has hundreds of items in her mailbox.