A West Highland Terrier with terminal cancer.
A parakeet came in for a beak trim and died in my hands during the process. I had heard of this happening for years but never experienced it. It was not healthy to begin with - was extremely skinny and turned out to be eggbound also. The owners were actually less upset than I thought they would be but at that moment I was very grateful to be the tech and not the doctor who had to break the news to them.
A five year old miniature dachshund that had jumped off the couch and broken its back. It was obese and most likely would not have been injured had it been in normal shape.