Long after dark one night I came across a cat that had been hit on a desolate road. I got out and picked it up. The poor thing was howling and covered in blood. Unfortunately, I couldn't see it very well to know the exact nature of it's injury. I could only feel the blood and smell the feces that it was covered in (from writhing on the pavement). I was on my way to meet a friend in a small town, so I called ahead and had him find a emergency vet.
I got there just before the vet did, and on the porch of the clinic I could see it's injuries... It's jaw was broken and hanging limp. The cat was obviously blind from the head impact (it's eyes were partially ruptured). Blood coming from the ears. A broken leg. Who knows what else. It looked very young, but I hoped that it could be saved. Even blind cats can lead fairly normal lives.
The vet, after looking it over and keeping it sedated for a few days was optimistic that the cat could be saved. I discussed the situation with the girlfriend for quite a while, and we decided to keep it (and foot the bill that would be enormous).
But, I told the vet to check it first for every major disease, since I have other cats I didn't want infected.
Turns out the poor thing was NOT a kitten. It was an adult with Feline Leukemia, which is highly contagious and untreatable. After all that, the poor animal still had to be put down. I was still out quite a bit of money as well.
And you know what? I'd do it again.