EOSINOPHILIC KERATITIS
When Mosaic was a baby, she had an infection - probably caused by a herpes virus - in her left eye. This type of infection is pretty common in cats and if untreated, can cause major problems. We've had two other cats with the same type of infection. Both were stray kittens when we found them and the infection had gone untreated for some time. Penelope ended up losing an eye and Mold (Molly Maguire) lost the vision in her eye. 
Knowing from experience that the condition needed to be addressed right away, we took Mosaic in as soon as we noticed the infection and started her on medication immediately. In spite of that, the infection, though it cleared up quickly, left a noticeable scar on her eye. Over time, the scar faded to the point at which we could barely see it and we thought no more of it.
One year later, her left eye looked goopy and irritated again and it looked like there was some kind of swelling on the eyeball itself. Figuring she'd had a relapse of the same infection, we restarted the medication right away. This time it didn't help. 
We took her to the vet, where we learned that instead of a recurrence of the original infection, she has an autoimmune condition called eosinophilic keratitis. This condition causes clusters of eosinophils (a type of white blood cell) and other blood components to form in the eye. No one is exactly sure what causes it, but it is often associated with herpes virus infections. 
The vet sent us home with some eye drops that contain steroids and the condition responded quite well to them and cleared up within a week - only to return a few weeks later. It responded quickly again, and returned again. Unfortunately, this type of scenario is common with this condition and some cats require treatment off and on for their whole lives.
On the plus side, it doesn't seem to bother Mosaic much and she's become much more cooperative about getting eye drops.

As an odd side note, all three of the cats we've had with herpes-based eye infections have been blue cats. I asked the vet if this type of infection tends to be more common and/or more severe in blue cats, but she'd never heard of that being the case. Perhaps it's just an odd coincidence.