[ His Watson deserves so much more than he gets. ]

Observation is a must unless someone is lacking morals and cares not for who else may ingest the poison. While it was an unfortunate accident, a chemical factory once poisoned many through the water supply, and there are those who will damn a whole town just to get revenge on one man.

[ Sherlock hopes for that the chemical factory has cleaned up the water from that particular contaminant by now. The other things that poison the water... that will take a much larger effort. ]

There is also a far more subtle method of poisoning. Instead of a full dose of poison, the poison is introduced little by little over a length of time. It often gives the appearance of someone falling ill because the changes are not sudden and harder to detect.

[ It's very good that the coffee machine is practically foolproof because had it been a normal kettle and coffee pot, the coffee (or cookware) would have been ruined in some manner from Sherlock's lack of attention. As it stands, the machine is just idle. ]