Replacement of fat by what? Or did you mean medullary replacement with fat? Sometimes atrophic kidneys have shrunken cortex so there seems like there is more fat in the area of the medulla, even though there isn't really. You can also get an angiomyolipoma that is mostly fat, and if you don't realize it's a nodule it can look like fat replacement.
Septic patients often have sloughing skin at autopsy. I think sepsis accelerates the process somewhat, so they get more obvious (and earlier) postmortem blisters and sloughing.