It's a sad end to a long happy life, but you did everything you could for your cat. I would like to say it is unbelievable that such mistakes could be made, but after years of working with animals and their owners, it's a story I hear quite often. We put our faith in the vets, because we have nothing else. Often people are labelled as over fussy worriers, when actually they know there is something wrong with their animal. Sometimes with older pets it seemed like the atitude is well it's old what do you expect.