Geoffrey House, M.D., drug abusing television show character, update of Sherlock Holmes, and Uber-diagnostic magician says, "People lie." When he's trying to help them and needs information relating to the personal world of their medical history, relevant actions and symptoms he doesn't always (usually?) believe them for that reason.
When patrons come in certain that such and such an item isn't on the shelf where it claims to be, I will look up the item record to see when the item last moved. If it was recently enough that we should still have it in the collection, I'll ask if I may go check the shelf and see if it was misshelved. (Because they just told me it wasn't there, and why do I doubt them!)
Another variation on the theme happened when a patron came in and told me that we don't have a book she used to come in and use. "Why can't you buy a real book that people use instead of all the novels that aren't real. I don't see why my tax dollars are going to buy stupid novels when the book I need isn't here." The book in question was Court Rules. She wanted me to get the web address for Court Rules so that she could find the information she wanted. (And didn't I know that if everything went to the Internet, we wouldn't need libraries any more because who needs to pay for a nice building if everything is online?)
Well, I gave the patron the web address and she took it out, reminding me that at this rate it wouldn't be long before we don't need libraries any more, think about it!
I decided to look on the catalog, just for my own edification. WE HAVE THE BOOK!!! I expected the larger branch might have it and the patron didn't want to go there, and as it is reference it wouldn't want to come here, but we actually have the book! If only I had looked anyway. If only she had asked if we had it instead of rebuking me for not having the book, I could have found it. And of course, she was gone from the building and gone from the parking lot when I checked out there, just in case.
Like House says: People lie.