Sometimes the text cursor stops flashing, and when I type nothing shows up. Then I click the mouse, and suddenly everything I just typed appears, and the cursor's frozen at a new location.
When I click inside the white area of the scroll bar, the page jumps and the cursor starts flashing again. Problem solved temporarily.
This started when I upgraded from 10.5 Leopard to 10.6 Snow Leopard. The answer to this question suggested that it was a mechanical issue with the mouse. For me it isn't, since I'm using the built-in trackpad.