Comment 12 for bug 970038

I can reproduce it 90% of the times using the attached html file in both Chrome and Firefox (just one pre tag with enough text to show the horizontal scrollbar).
In my case, scrolling it to the right and all the way back to the left, so that the mouse pointer is in the panel's area, cause unity panel to fail to reveal.

As I also found out, it doesn't have to be a web browser - text editor like GEdit will result in the same behavior, when a line is long enough to reveal the horizontal scrollbar.