The location of a badger latrine that I had previously been unaware of. Needless to say the dog found it before I did. I had to try to get him to walk home downwind of me. I then had to bath him, of course, in full PPE.
EDIT: I have since learned, from the dog trainer we see each week, that it would have been fox poo, and that fox poo is the WORST. I was directed to a local supplier of industrial strength dog shampoo, but since he's been out in the rain a few times since his bath the pong has mostly been mitigated. But the fox-poo shampoo stands in readiness for the next inevitability.