I'm not sure if this was posted somewhere else or if it's a known issue, but i've noticed a small bug in the size of the taskbar preview.
When you hover a taskbar button the size of the preview is normal for a split second, and then it shrinks on the bottom side for a few pixels.
It's much more noticeable on previews that have buttons (eg. multimedia apps)
It only happens when the taskbar is set left or right,