This happened a few years ago, while I was staying at the guesthouse of a friend. I'd go upstairs for a game of backgammon, occasionally take a leak, and marvel at the state of her toilet.
The toilet, 70's porcelain, peach or flesh colored with a fur cover and rug around the base. Inside, below the waterline, grew a thick matt of algae, black. I marveled that she never cleaned it, and good manners forbid pointing it out (it was obvious, after all). I'd check on it whenever I was up for a visit and had need to pee, half expecting that that stream of urine would dislodge some fauna, crabs or starfish creeping around the bowl...
...now, a disclaimer, as they were on septic they argued against the use of harsh chemicals to clean their toilet. And - as I am a long way from even being a mediocre housekeeper, well, I'm not in a position to throw stones.
Anyways, one day, up for a visit and I have to lay some pipe. A good, 2 foot long very solid turd. And, bidding it goodbye and flushing the toilet what should happen but the turd should rise up on the waves, swirl round the bowl and in one smooth motion tear the entire algae/matt off and take it down the whirlpool...
I left the toilet shouting for my friend, able to keep my silence no longer: "...my SHIT just cleaned your TOILET...!!!"