You're supposed to be able to just stop moving the mouse, wait a second, and it will correct itself. Occasionally the PS2 protocol skips a byte and the mouse driver gets confused. This used to happen to me on a laptop with a trackpad, especially if the trackpad got zapped with a static electricity shock when I went to touch it.
The source is in /sys/src/9/pc/mouse.c if you want to tweak the logic that tries to get out of this state. Or you could plug your mouse into a USB port. Russ
