An author makes arrangements with a bookstore manager for her to come in and sign several copies of her books. It's not a formal book-signing event. She'll just sign the stock and they'll put it back on the shelves with a note that the books have been signed by the author. It happens all the time.
Here's where it the story takes a turn from the usual.
Apparently, the manager didn't tell the store clerk. The clerk thought the author was vandalizing the books and wouldn't let her sign them without buying them first -- which defeats the whole purpose of signing them. In this case, the author left without signing her books, and the bookstore was stuck with extra unsigned stock that they'd ordered in especially for this signing.
A local Chicago-area author (I can't remember who -- sorry) told this story last week during a discussion on publicity and book signings at my writers group meeting.
With this story fresh in my mind, I laughed today when I saw this headline about Stephen King in a similar situation (only he apparently didn't arrange the signing with the store manager):
Horror author Stephen King mistaken for vandal in Aussie outback.