Pen looked a little off to me one day.  There was nothing specific going on, really: he just seemed off.  I took his temperature and checked his color, and everything appeared normal.  I called the animal hospital at which I worked, talked to one of the vets, and got teased about being a paranoid mother.  I had to agree the vet was probably right, especially when Pen appeared to be fine the next day.  He'd had a thorough exam by a vet less than two months before, too, and had checked out just fine.
The following morning though, he was obviously depressed, so I took him in.  His temperature had dropped below normal and x-rays and other tests yielded a diagnosis of dilated cardiomyopathy (enlarged heart), a serious, but sometimes treatable disease.  We started him on medication, but he died suddenly 24 hours later.
Losing him was horrid and it took me a long time to recover.  It's always awful when a pet dies, but losing Pendaran and - many years later - Arivaca and Penelope, was harder than usual, maybe because they were so young and died so suddenly.

A couple of months after Pen died, one of the vets I worked with showed me an article about feline cardiomyopathy.  Researchers had just discovered that a lack of the amino acid taurine in cat food was often a causative factor in the disease.  Since that discovery, pet food manufacturers have increased the taurine content of cat foods and the incidence of this disease has declined.  Hypertrophic cardiomyopathy, unfortunately, is still around, but many cats can live with it for some time with appropriate treatment and medication.