A problem that needs to be contended with if you own a Rangefinder or Twin Lens Reflex camera. Namely - your eye is viewing the scene from a different point of view from the lens. You can simulate the problem of paralax by closing one eye and pretending that the closed eye is your camera's lens. Now swap which eye is closed and watch the resulting 'jump' that occurs as your visual perspective changes. This can result in all sorts of minor changes and can sometimes be used as an excellent excuse for why did did a terrible job of composing that last photo...

"Damn Rangefinders, if I had a real camera I never would have cut off Lucy's head!"

Sounds nice and all, but for the most part the actual alteration of the image is very slight unless you're shooting a subject that is closer than two feet.

"But what if I wanna shoot something closer that's right in front of my nose?" you ask...

Well, if you own a camera that suffers from paralax: don't. It just wasn't made to do it. Go buy yourself a Single Lens Reflex, or better yet, a View Camera.