"The new scrollbar in Karmic's Human theme has a usability bug:
To reproduce, open nautilus, maximize the window, then open a directory with enough files to cause the scrollbar to appear (e.g. /lib). Now move your mouse to the right edge of the screen, and left click. Instead of grabbing the scroll bar, the mouse will behave as if you had clicked the scrollbar background, i.e. the bar will jump to that position (in Firefox, nothing happens). The problem is that the bar doesn't extend to the right edge, leaving a two pixel margin, but because of Fitts' law, it's much easier to click on the screen edge than the few pixels to the left of it.

The cause of this is GtkRange::trough-border being set to 2. Setting it to 0 removes the margin around the bar."

From Launchpad: https://bugs.launchpad.net/ubuntu/+source/human-theme/+bug/422511