I want to complain, but I have nothing to actually complain about!
1. I've got it right here and I'll bring it back straight away.
2. I didn't know that was due, I'll bring it back as soon as I can.
3. I've never had that book in my life.
4. I've already returned it, please check your shelves.
I always use a cheerful smiley-smile voice, as I don't think anyone wants to deal with a grump.
But today! Today one borrower said she didn't know they were due and she hadn't seen any email notices and she thought the fines were unreasonable (.25c per day, per book). I told her we'd sent 3 email notices, and it was really up to her to keep an eye on her library account. She snapped that she would bring them back as soon as she could, and ended the call, rather abruptly I thought, but no-one likes to be told they owe money, so I left it and moved on down the list.
About half an hour later, my manager was called out to the front desk. She came back and said that the borrower had been in and complained about me. Her exact words were "I didn't like her tone!"
My manager told me that she'd often heard me on the phone and she knew I wasn't rude to people, so she just calmed the borrower down (and did NOT waive her fines!) and sent her on her way.
But this phrase is a masterpiece of the special snowflake genre. It can be used to complain in a situation where there is no basis for a complaint! There's basically no answer except an apology and (if they meet a soft opponent) a backdown. I just hope the word doesn't get out in the mofo community (there must be one, they can't all be operating independently!)