Lord Samphire
Referenced in Nanny Ogg's Cookbook by Vetinari, as a previous post-holder who got dangerously complacent about food additives in his diet.
Samphire was astute enough to check that neither the bread nor the water was hazardous on its own. However, as Vetinari notes, "the problem only came to light if you ate the bread and then drank the water".