Scott Pack writes here: Me And My Big Mouth :: My Christmas Angel Moment about how he was able to help the man in front of him in the Waterstone's queue by providing the title and author of the book the man was trying to buy (without remembering the author or title, a frequent occurrence in bookshops as has been noted here.) The book, by the way, is The Woman in Black, and the author Susan Hill.
I had a similar experience the other day: one of those rare occasions when my day job came in useful "by chance". I was walking up the stairs at Wimbledon station in a mass of coummuters when two young women behind me began to talk about their upcoming chemistry exam. "I can't even remember who won the Nobel prize this year", one of them lamented. I was able to turn round and enlighten them. I think they were so surprised that the name would have stuck in their minds at least until their test came up. (The answer is Roger Kornberg.)