This occurred to me when I was watching my friend using my computer (F16 + GNOME 3.2).

Situation:
* Firefox is opened in full-screen mode

Action: Move mouse to the bottom right edge of the screen to press the little arrow on the scroll down.

Planned outcome: Pressing the left mouse button scrolls down

Actual outcome: The message-tray (empty) pops up. She clicked numerous times and nothing happens. She moves the mouse back and forth a few times. At some point she happens to be as far away from the corner to be able to scroll.

This was very interesting to see especially because the figured out how to switch between windows and how to start apps pretty quickly.

(This never happens to me because I almost always use the mousewheel to scroll).

Possible solution:
* Use the bottom right corner as hot corner
* Increase the speed / time needed to access the hot corner
* Leave a gap in the corner of the message tray where clicks are actual going "through" the message tray