EPILEPSY
When Jessamyn was about a year and a half old, she suddenly began having episodes during which she squatted on the ground, yowled, and urinated.  She was very clingy and affectionate before and after these episodes.   This happened numerous times in the course of the day and we figured she had a urinary tract infection.  We took her to the vet and sure enough, they found a mild urinary tract infection and started her on antibiotics. 
She continued to have the squat/yowl/urinate episodes after starting the antibiotics and we kind of laughed at her for being such a weenie about a mild infection.  The day after she went on medication though, I came home from work to find Chris rushing out the door with her in a carrier.  She'd had a grand mal seizure.  The vet figured it was caused by an allergic reaction to the antibiotic, so we switched medications.  She then had another grand mal seizure late that night and continued doing the squat/yowl/urinate thing, so she was hospitalized the next day for tests.
At this point, we put two and two together and realized that the squat/yowl/urinate episodes were not an indication of a urinary tract infection, but petit mal (also called partial complex) seizures - and she'd been having up to 15 of them per day.  A lot of cats have petit mal seizures rather than the more obvious grand mal ones.  Petit mal seizures often consist of odd behavioral changes, such as Jessamyn's random urination, rather than "typical" seizure activity.  The urinary tract infection was just an incidental finding and may not even have been a true infection, since cats tend to have a lot of bacteria in their urine. 
Tests showed nothing abnormal, so she was diagnosed with idiopathic epilepsy and started on phenobarbital, an anti-convulsant drug.   Idiopathic epilepsy is the diagnosis you wind up with after you rule out all other causes of seizures, such as organ failure, infection, tumors, and the like. 
After experimenting a bit, we stabilized her on a fairly low dose of phenobarbital and she was seizure-free for years. She also gained quite a bit of weight, which may or may not have been related to the medication.  In early 1999, we made the decision to try to take her off phenobarbital to see if that would help with the weight problem.  It didn't, but she was seizure-free without medication for the rest of her life.  In addition, she was obviously less drugged and was a bit more active.
Maybe someday someone will figure out what causes epilepsy and why it sometimes just goes away.  I'll always wonder if Baby's epilepsy had anything to do with her very poor physical condition as a kitten.  Perhaps she didn't receive the nutrition she should have, even in utero, and something didn't develop quite right in her brain.  Whatever the case, we're just grateful the seizures went away.
RATTLESNAKE BITE
Jessamyn managed to get bit by a rattlesnake, despite being a strictly indoor cat.  Naturally, this happened right after she was diagnosed with epilepsy and we were trying to determine the correct phenobarbital dose.  The snake apparently stuck his head through a hole in the porch wall around the dryer vent, and she batted him and got bit in the right front paw. 
We had no idea this had happened, but I noticed her paw was swollen and, figuring she had an abscess, dropped her at the vet's before work.  We were quite surprised to hear the diagnosis, needless to say.  A little anti-venin and lots of $$$ later, she was fine.  Cats tend to do better with rattlesnake bite than dogs and people do, fortunately.

Apart from being fat, Jessamyn managed to stay out of trouble for quite a few years after the seizures and the rattlesnake bite, thank goodness!
DIABETES

So much for staying out of trouble...  In late July of 2003, Jessamyn starting drinking and urinating in startling quantities.  I was worried that her kidneys were starting to go, but blood glucose and fructosamine tests revealed that she was diabetic.  After some experimenting with the dose, she was stable at 3.5 units of insulin twice a day. 

For a week or so, Jessamyn was our second diabetic cat, but while starting her on insulin, we got some good news about Irene, who'd been diagnosed several years previously.

ARTHRITIS AND HIP DYSPLASIA

In March 2004, we took Jessamyn in to the vet because she was walking oddly and acting like her back or hindquarters hurt when she got up and down.  The vet took an x-ray, which told us three things:  she had severe arthritis in her lower spine (two of the vertebrae had fused together); she had hip dysplasia; and she had an enlarged liver.  (The enlarged liver is addressed below.)

She almost certainly had hip dysplasia from birth, but like most cats with this problem, she had compensated for it, so it went unnoticed.  Her weight and the arthritis in her spine lessened her ability to compensate.  We treated the hip dysplasia and the arthritis with steroids for a while, but switched to a new pain-killing and anti-inflammatory drug called Metacam to avoid further stressing her liver.  It helped a good deal, though she was never 100% normal.  

ENLARGED LIVER

When we took the x-ray in March, I was expecting to find something like arthritis.  The hip dysplasia was a surprise, but didn't seem to bother her much.  The enlarged liver was not good news at all, and her liver values were not normal, either.  The two most likely candidates for causing the enlargement and the out-of-whack liver values were cancer and hepatitis.  We treated her for hepatitis, but it didn't seem to make a difference.  A liver ultrasound did not show any tumors.  Some additional bloodwork showed that her pancreas was inflamed, which may have caused the liver enlargement.  We never did figure out why it was enlarged, but it didn't seem to do her any harm.

ACUTE KIDNEY FAILURE / BAD REACTION TO PIROXICAM

By December of 2004, Baby's arthritis was getting worse, so I decided to see whether she might be a candidate for surgery.  The surgical specialist said it could be done, but that there was a newer and better painkiller out there called Piroxicam that might be worth a try before we resorted to drastic measures.  That seemed like a sensible idea, so I started her on the Piroxicam the next day.

Fast-forward to three days later:  I checked on Baby before I went to bed and found her on the bedroom floor in a near-comatose state.  Initially, I thought she was dead.  Assuming insulin shock, I administered Karo syrup and sure enough, she came around and looked much more normal after just a few minutes.

She went to the vet the next day, where bloodwork showed signs of major inflammation somewhere in her system, plus elevated kidney values.  Piroxicam is known to cause GI inflammation, though it's not usually that severe.  I think her stomach had been upset, so she'd quit eating, which can be disastrous for diabetics.  I had seen her at the food bowls and assumed she was eating, but apparently she'd just been hovering over them, looking.  Needless to say, we stopped the Piroxicam immediately.

Kidney values can be elevated due to dehydration, so Baby got fluids, and when she perked up and started to eat again, she went home.  She did all right for a few days, then quit eating again, so she went back in and was hospitalized overnight.  Again, she perked up, and for a little while, she looked better than she'd looked in a long time, which made me very happy.

After a week or so, however, she quit eating again and started to look much worse.  She went into the hospital again on December 20th and was diagnosed with renal (kidney) failure.  Adding to the stress level was the fact that Obsidian had been diagnosed with kidney failure and Finian with osteosarcoma the month before.  

Despite intensive treatment with IV fluids and medication, she got worse instead of better and by the afternoon of December 22nd, it was obvious what we were doing wasn't going to work and she didn't want to try anymore.

There is no doubt in my mind that the Piroxicam caused the insulin shock episode and I'll never use it on another diabetic animal.  The kidney failure may have been a coincidence, but I strongly feel that the Piroxicam was related to that, also.  I believe Baby probably developed mild kidney problems after her last bloodwork and before we started the Piroxicam, but the symptoms were masked by her other problems (e.g., frequent drinking and urination caused by the diabetes).  The Piroxicam then made things much, much worse.

I'm sure Piroxicam is overall a fairly safe medication and I don't think the vet made a mistake prescribing it.  I think Jessamyn's case is not the norm.  That being said, I would advise anyone thinking about starting an animal on this medication to have bloodwork run first to check for both kidney problems and diabetes.  Better safe than very, very sorry.