When we were living in Ontario another fellow cacher found a dead human body close to a cache. I don't think it gets any worse than that. He posted about it on the groundspeak forums at the time.
We personally have found a cache next to a rotting coyote. The smell was terrible and we almost puked. That didn't stop us from signing the logbook though.
Worse thing we found inside a cache was in Ontario where the entire cache container was full of earwigs
. From that point on we almost always wear nitrite gloves when caching. We noticed worldcaching recently started selling these gloves.