My mother does the "babysitting the bookshelf" witnessing. She told me story of the cart being approached by an irate apostate. The woman was carrying on about something and the sisters asked her to show them. The woman said she couldn't. The sisters asked her to leave them be. She continues her rant until a lady watching came over and asked for a book. The lady said she had been watching the exchange and felt bad for the sisters so decided to rescue them by taking a book.
Goes to show that sometimes unwitnessing to cart watchers is not always the best way to go.