It's not something that happens often, but since I do write so much about our experiences fostering kittens, I should tell you that sometimes a foster kitten will die.
White Mittens came to us already ill, with chronic diarrhea. We medicated her, and kept her clean, and encouraged her to eat, and just as importantly we cuddled her, and talked baby talk to her, and carried her around in Sydney's doll sling:
Alas, poor babe, she lived with us for a week and then she died in the night. I can't say that she had a happy life, since she spent it all being ill, but she did have comfort, and a family, and two kids who loved her quite a lot.
Giving them a better end is just as important as giving them a great beginning.