I got a passive 3D monitor a few months back and I have noticed that at times I can see ghosting (crosstalk, a double image, whatever you want to call it...) in my games. I realize that most of this ghosting (if not all of it) is due to the quality of the monitor and glasses. But this led me to think of the following situation were a double image could appear due to the way the brain processes the image:

Lets say you have two balls rendered in 3D on the screen, the image for ball one appears 12" in front of the screen, and the image for ball two appears 24" behind it. When you look at the image your eyes will focus on the screen (since that is where the light is coming from) but your eyes will converge on the ball you choose to look at. So if you were to look at the front ball, the back ball would appear as a double image, and that double image will be in sharp focus since your eyes are focused on all light coming from the screen.

Now if you take the same set up in real life and you look at the front ball, the back ball will still form a double image, but this time it will be out of focus. Since it is out of focus the brain will just ignore that information.

Because of this would it be right to say you can never get rid of all ghosting regardless of how good your display is? Or are effects like this so small they would not matter?

Ghosting or, more correctly correctly, crosstalk is when one image that was intended for a certain eye "bleeds" into the image for the other eye. It has nothing to do with where you converge your eyes on the stereo image, or if all parts of that image are in focus. That is something else altogether. I mean, you can observe this in real life. Just put your index finger about 3 inches from your eye, but focus on your computer screen. Look. You will see 2 fingers. But this is not ghosting.

A small additional note since you mention sometimes: Sometimes certain effects in games are rendered in a mono, distorted or reversed fashion which can be mistaken for crosstalk. In this case it's because of faulty interpretation of the driver or just simply because the original effect does not have any depth (gamedevelopers didn't have stereoscopy in mind when doing the game).