In the book Hyperion, there is a creature called the shrike, which is an all metal AI killing machine. It is first said to have four arms, but through out the book, (and all of the books, for that matter) it keeps switching back and forth between 2 and 4.
I don't remember the books ever mentioning the Shrike only having two arms. Sometimes, it just doesn't mention all four. The cover art, interestingly, has the Shrike with only two arms. Hmph. Those artists never do read the books, do they? (Except for Michael Whelan.)