Are you a nitpicker? I certainly am. After all, it's all in the details, isn't it?
Some tiny little mistakes might not catch your eye immediately, but then there are those that jump at you in such an obvious manner that it really makes you wonder why some authors don't invest some of their time spent on planning and plotting to create a list with important “landmarks” of their story and characters. I make lists. Really. It helps a lot. And it ensures that a character will have those green eyes from the first through the last page and not suddenly impress anyone with dark brown eyes instead.
The websites Bookmistakes and Slipups are dedicated to all kinds of mistakes that can be found in books. Not restricted to characters with changing eye colors, but also details like cars with four doors which in fact should only have two (not as though I could actually name a specific model now, hey, I'm a girl, I distinguish cars by their color) or flaws in history (there is something called research, argh).
Here are just some of the mistakes that will (or maybe not) make you chuckle (unless you're the author, than you have every right to cringe):
From Stephenie Meyer's Twilight
When Edward is telling Carlisle's history to Bella, he says that Carlisle actually found a coven of true vampires that lived hidden in the sewers of the city. The vampires couldn't have been hidden inside the sewers because there weren't any. The sewage system was only built around 1859, and Carlisle found the vampires around 1660-1670.
From Stephen King's It
Eddie's broken arm changes around, sometimes it's the left arm that's broken, sometimes it's the right.
From Ray Bradbury's Fahrenheit 451
In the first part of the novel, when Montag is thinking about Clarisse, it says that he sees her shaking a walnut tree every once and a while, but hen it starts talking about how she leaves chestnuts in a basket on his front porch, and how she was rattling a chestnut.