if this does not happen to your phone you are in the happy minority of nexus one users.

to the rest of us, while using the touch screen for extended periods of time (shorted if using both fingers and/or charging) it will start to behave erradicaly. Touching one area will register a touch somewhere else. This is also affected by the pressure. the harder you press the less likely to see the bug. It will get worse and affect larger areas until you turn the screen off and on again (lock, unlock and it's back to perfect condition)

worth to mention, this does not happens with some mods. but i haven't rooted my phone as i use it mostly for development and like to test against the stock image.