Our first kitty, Samantha, started hiding behind the couch more and more, not eating, and a trip to the vet revealed some kind of tumor on her ear that I'd hope we would've noticed if she wasn't always hiding. We also had a newer kitty, Josephine, who was a typical tabby and full of energy, so Samantha hid behind the couch to get away from her, and we didn't realize at first that Samantha was just plain not hanging around at all. She basically spent all her time behind the couch. We scheduled surgery to remove the tumor, which was malignant, and they were "pretty sure" that they got it all, but you can never be 100% sure. Or so they told us. Our son had been born, my new job was not panning out the way it was supposed to, and my wife was having trouble finding a job after taking time off to have our son, so money was very tight at the time.
After the surgery, Samantha was better for a while, but after a few months she started hiding behind the couch again. The tumor had grown back. Newborn son, a total of five mouths to feed on one income, and having another surgery would really hurt us financially, so it was pretty much out of the question. We put food and water behind the couch so she could stay back there all the time, which I'm sure sucked, but somehow putting her down seemed even worse. One night she came out and my wife picked her up and carried her into my office crying, asking if there was any way we could afford the surgery. Well, the vet takes credit cards, and what the hell, it's only money. We decided to call the next day to schedule another operation. Samantha died overnight. She had come out the night before to say goodbye to us. That's the only thing that makes sense, as she'd stayed back there for a month straight, but came out for some last pets, then passed away.