Ever since I installed the 10.6.4 update my mouse started jumping randomly across the screen – often into active corners, starting the screen saver.
At first I thought the batteries were empty or the mouse needed cleaning, but that didn’t help. Neither did using a mouse pad.
It seems this is not an isolated issue:
I even connected my Logitech Laser Mouse (which I’m going to use in the future instead of my MagicMouse – it’s just too good…), and it shows the same identical issue.
UPDATE: It seems to be connected to the Magic Mouse after all – when I tested the Logitech I had the Magic Mouse still switched on and connected, too. Once I switched the Magic Mouse off, the pointer behaved well again.