Many time when I move the mouse over the taskbar (I use 2 lines taskbar) the tooltip appears under the mouse, not above the panel. In such cases you cannot activate the app you want by clicking on it, as you'd click on the tooltip instead... To fix it, I have to move the mouse over another app, so its tooltip appears (now correctly). From that point it works, but if I wait some time, like the time I needed to type this bug :), the bug appears again.
*** This bug has been marked as a duplicate of bug 207513 ***