It took me quite a few times before I stopped loaning knives altogether.
The first time, I left a knife at a friend's house by accident. He proceeded to use it, and then tried to make it up to me by sharpening it...
...On an angle grinder. Luckily, it was just some cheap fixed blade that I purchased basically entirely for looks. We were both 15. It was forgivable.
The next time, I lent my Counter Point II to a friend at work, and she proceeded to try to cut metal straps with it. Had to sharpen some dents out.
Same coworker, a month later, I lend her the same knife. She leaves it in the open and locked position, unattended, on a shelf in a retail store, for over fifteen minutes.
Now she complains that I never lend her my knives. This exchange happened verbatim:
Her: "You don't think I'm responsible with them! You think I'll lose them! You don't trust me with anything!"
Me: "Exactly. You've never given me reason to."
This is why you don't work with exes. This is the same person that had FIVE cell phones in a year because she kept breaking/dropping them, and has a daily panic attack trying to find her keys. Sigh.
Anticlimactically, the next one is pretty minor - I took a coworker's knife home to sharpen it, and I lent him my Enlan El-02B just so he had something to use while I was sharpening the knife (our work schedules don't match up too well, so it was going to be a couple days before I saw him again to give his back)
I bring him back his knife, having given it a spa treatment. Took it apart, cleaned it, lubricated it, sharpened out some chips, and gave it a polished, razor-sharp edge. He hands me the EL-02B, and it's covered in dirt and gunk, has chips all over the blade, and now the lock sticks.
I don't loan knives anymore. To anyone. For any reason.