If you're a hobby photographer like me, and you've had a chance to play a bit with panorama photographing and stitching, you've probably noticed that it's very hard to avoid the so-called parallax errors. This is especially the case if you're shooting by hand, and some of your objects are relatively close to the camera while others aren't.
During the stitching process, parallax errors will cause some parts of the images to fit together with each other, while other parts are off. There are very technical descriptions of this phenomenon available, so I won't go into any kind of detail about that here. The important thing to avoid it, is to rotate the camera around the no-parallax point in the camera's lens body.