Not all copies of the same comic are created equal. Some are minor differences, like cover centering being off, but some have differences on the inside too. Take this panel from Marvel Super Heroes Secret Wars 1, in which Galactus isn’t his normal shade of blue and pink. This is known as the “Blue Galactus” error edition. It was likely caused either at the time of printing, or the colorist forgot to color that part of him.
This isn’t the only instance of this happening in books, and it’s a cool niche genre to collect. Some increase value by quite a bit, so always be sure to be acquainted with any possible errors on a book you are interested in!
In a similar vein, Recalled books have gathered the attention of collectors as well. Some are pulled due to a graphic image, some questionable verbiage, or even colors being wrong (like the example above, though the Secret Wars book wasn’t recalled AFAIK).
As always, check out the video below for more info!