It was like that when I got it

One of my favorite patron comments, aside from the obligatory "I know I returned that book because I keep all my library items (insert on a table, in a basket, on a shelf next to the door, etc.) and never take them anywhere else," would be "It was like that when I got it."
You can tell when someone is going to pull out this excuse, they usually have a bored expression on their face that translates to "I don't care," or "I can't hear you over the shouts of my 8 uncontrollable children," or "I could stab you and hide your body in a crawlspace that no one would find until it began to stink." Visually, these people look like they may also have a guest spot on People of Walmart, and they never smile. Yeah, I'm sure it was like that when you got it.
What I would really like to say in return is, "Take some adult responsibility and pay the $9.99 for the Tom and Jerry DVD that your toddler snapped in half." Instead, I have to go through the game where I explain that each item is checked upon return and that it could not have been like that when they got it, even though we both know what really happened.
According to these people, my branch circulates DVDs and music CDs that are not only broken in two, but books with large chew and tear marks, books with covers pre-ripped off, DVD cases that have bits of moldy food in them, Hooked on Phonics kits that are completely empty, magazines with entire sections cut out, and, in two separate cases, books that were returned still completely soaking wet (dripping is an understatement) that were all miraculously like that when the person checked it out.
Now, I realize that the library isn't perfect, and that sometimes an item is returned damaged and not caught, but then wouldn't the next borrower notify the library as soon as they noticed a problem to ensure that they aren't held responsible? Or at the very least, bring it to the circulation desk and talk to someone about it, instead of just putting it in the bookdrop like nothing happened?
What prompted this post was a recent incident when a patron who claimed a book that had been marked as returned damaged by her was like that when she got it. The item in question: Liar, liar! That book's been on fire!
The book was actually ashing on my desk and the binding was so damaged that the cover was falling off. You might ask what other detective skills I used to confirm that she was the guilty party? Well, the dust jacket had been suspiciously taped on and the book was practically brand new, only previously checked out twice. She ended up paying for the book (an outrageous, bank breaking $23), but let us know she wasn't happy about it. Lucky for her, I didn't care, though I was curious to know what had happened to this book, but I suppose it will just go as an unsolved library mystery.
I only wish I'd had the staff digital camera around the day that the teen returned a book that had been put in the microwave.