I think it's the case where the water supply (out of the wall) had an open hose that dropped down into a basin and at the bottom, made physical contact with a resting pool of cyanide-containing fluid.

Capillary action could possibly induce the cyanide fluid up into the hose, and/or backwash/turbulence could mix fluids inside the hose, and in just the wrong conditions, could result in some cyanide back-flowing up to the piping, to be distributed elsewhere in the building. As an extreme layman, I'd assess that as unlikely but at least possible.