That one time Holmes used the teapot to experiment with a certain egyptian poison and it absolutely peeves Watson:
“Is nothing immune in this place?!?”
“Oh nothing to worry about, Watson. It’s perfectly soluble. I’ll have the pot washed out and ready for use by the time you come back from Regent’s park.”
There’s a scene in another episode where he’s keeping tea in a jar marked “SNAKE POISON.”
(via thenorwoodbuilder)