Here's a case I had a few years ago, and one that I'll never forget.
Someone brought their 4-5 month old kitten to see me because it had something hard in its throat. The little girl was a very sweet cat, and apparently was acting perfectly normal at home. The issue was pretty obvious at a quick glance. From the front of the cat's neck there was a small but very noticeable projection just under the skin. When I felt it, it was firm like bone, and could move slightly from side to side. When I tried to move it a little, the cat acted like nothing was wrong. Frankly, I was very puzzled. It felt like a bone, but there are no bones in that part of a cat's neck, and the cat seemed completely unaffected. So I recommended x-rays to the owner and they agreed.
This led to my first surprise of the day. The object that I had been feeling was a sewing needle! Somehow the cat had swallowed it, and half-way through the esophagus it had turned 90 degrees and was poking through to just under the skin. What really frightened me was that the pointed end of the needle was only a couple of millimeters from the kitten's spine!!!! If I had pushed to hard on the "object", I could have accidentally rubbed it against the vertebrae, or even gone between them. Yikes! And the kitten acted like nothing was wrong the entire time.
The kitten was old enough to spay, and I would have to anesthetize her anyway to remove the needle, so the clients agreed to do both. I put her under anesthesia, and made a small incision in the skin just over the needle. Once through the tissue, I was easily able to grab hold of it and start to pull it out. That's when I discovered surprise number two. As I pulled it completely out of the kitten's neck, I noticed that there was string still through the eye of the needle. I kept pulling and thread kept coming! It seemed like there wouldn't be an end to it! Eventually I removed about 18 inches of thread, along with the needle!!! All of this from a 4 1/2 pound kitten!
I sutured the incision closed, and proceeded to do a routine spay on her. She recovered normally, came back 10 days later for a post-operative exam, and everything was normal. Despite all that she went through, she had no long-term adverse effects. A very lucky kitty!
Lesson #1: Everyone be very careful with thread and needles around your cats.
Lesson #2: You never know what you're going to see when you're a veterinarian!