The disease that seems to give corys the most trouble is bacterial infections. This can lead to barbel erosion as well as finrot and sometimes death with no visible warnings. It can happen either quickly or slowly depending on the strain.
It seems to me that my C. pandas are more apt to be infected than the other corys I keep. Even though they are in a tank by themselves, and that tank gets cleaned at least twice a week, I've had to treat them for it.
Fortunately, the strain that shows up in that tank looks terrible, but seems to respond to little more than a few days treatment with MelaFix.
Any gravel that lets uneaten food fall into the cracks where the corys cannot get to it, can result in a sudden rise in the population of harmful bacteria which can cause the corys to get infected. This alone is a good reason to avoid large, pebble like gravel.
Erosion of barbels seems to be more likely to happen due to disease than to keeping corys on large gravel, but too, this only happens in advanced stages of the infection. Most often the fish die before that happens.
and then i found him literally belly up 
