Even the best shelter is not a home, or home, or the place of a loyal human-animal bond.
Even the best shelter can have problems with disease, or be a stressful place for a particular animal, or just not be able to bring the resources and conditions of home to bear on a problem. Both shelters I am involved with have fallen short in the ability to care for one or more animals with the result that the animal died. Not in the sense of wrong doing, bad practice or neglect--in fact, many of these cases the shelter went above and beyond--but in the sense of just being a crowded, busy, germy, noisy place that is not home, no matter how nice it looks.
I was looking for pictures but I am just going to name names for now.
Brick--for being in the wrong place at the wrong time.
Jasmine--for getting too sick
Onyx--stressed too much in the kennel
Maria--got too sick and stopped eating
Three little kittens--for coming to the shelter too young, and in PanLeuk season
Kittens too small to vaccinate, born of unattended mom cats, etc--fading away for so many reasons.
and finally, even though it isn't officially common knowledge...
Evelyn--undone by the continuous change presented by being in an animal shelter. Even the best behavior mod program has trouble overcoming the simple fact that it is not home, with a consistent human cast and physical environment.
Rest in Peace all you cat souls. For me, at least, you did not pass away without notice.