What is the point? Do they think that they are the only reader whose eyes and mind will detect the mistake? Really! Anyone who chooses to read the book they have seen the need to correct will either consciously notice the mistake, or auto-correct it as their eyes race across the paragraphs. What monstrous ego compels an otherwise well mannered library patron to so needlessly deface a book?
What if the "error" is not a mistake, but a deliberate choice of word by the author that was intended to convey some special meaning in a poetic way? What if it is poetry, and has confounded the literal minded volunteer editor. What would such an editor have done with the works of Shakespeare? Would any copy of the works of Lewis Carroll remained unmarked by such enthusiasts?
As library staff and patrons know, these pen wielding pedants make more murk than clarity. They should annotate and sign for every correction, on a flyleaf or insert, so the rest of us can gloat if their correction turns out be incorrect. They should allow for the likelyhood that other readers who pick up this book after their pen has made its mark will share (or even surpass) the volunteer editor's facility with words, and refrain from such petty, pedantic scratchings.
Sure, if it is an "early reader" that is meant to teach a small child the correct spelling, use, and meaning of words, by all means show the book to the librarian at the circulation desk when returning it. Perhaps you can write a letter to the publisher or author setting out the mistake, but for those books aimed at literate, experienced readers, please stay your hand and set down your pen. If knowing the "error" is there is keeping you awake at night, please show it (un-annotated) to the librarian.
But as annoying as the volunteer editor can be to librarians and patrons, there is someone who will be consigned to a deeper circle of library hell, and that is the self appointed censor.
When all the books on a sensitive topic suddenly vanish, are defaced, or are found kicked under the bottom shelf, the librarian knows someone's cherished beliefs or opinions have been wounded. The librarians care, they really do, and it is possible that the books in question may offend the librarians, too - but modern public libraries commit to free and open circulation of books and information, and to not censoring publicly available information - despite the fact that more than a few of the books that are officially catalogued as non fiction really deserve to be put on the paperback fiction shelves, with a "Fantasy" genre sticker on the spine.
To those few who feel they have the right to censor our public libraries collection, please have a polite chat with the librarian - there is a policy they must follow to determine if the book should in fact be on the shelf. If the policy does not give you the result you seek, please take it up with the author and publisher - perhaps a word from you could set the author on the right path after all. Just don't take it out on your public library.
Public libraries are, after all, one of the pillars of a free and democratic society - if you doubt that, then ask the librarian to direct you to the shelves containing books on politics, society, and history.
Spend a few weeks or months reading deeply on the subject and you will soon find out what sort of society results when the censors move in and the books are destroyed or burned.