I use a dual-monitor setup with the primary monitor on the left, and where the primary monitor has a smaller vertical resolution than the secondary monitor. This means that what is supposed to be a hot corner for the message tray is actually a few pixels hanging in the the middle of space, making the message tray very difficult to activate.

This problem also affects the workspace add/remove buttons in the overview, since they're supposed to be a "hot side" but on my setup are just a thin strip of pixels hanging in the middle of the screen.

It seems like the simplest solution would be to add some edge resistance for the cursor around the edges of each monitor. This would have the added benefit of making it easier to use scrollbars, etc, on windows that are maximized on the primary monitor.