Let's see... was it the case of explosive diarrhea that painted the stall, the back wall and the *ceiling* but left the toilet untouched?
Maybe being tasked to clean out a dorm kitchen at a university and discovering that the contents of the fridge had achieved sentience and was in the process of migrating out into the new world?
Nah, first place goes to the day that I had a customer come up to the returns counter at Wal-Fart with a large bag of clothing and a fistful of receipts.
A bag of lingerie. All of it very much worn before and never, ever washed. Ever. I won't even try to describe the smell. I am still thankful that it's actually a LAW here in Canada that undergarments cannot be returned for any reason beyond obvious defects, and if they've been used not for any reason at all.