Never trusted corporate coffee since my schooldays, when I conducted a terribly childish vendetta against the 6th form capitalist running dogs who were the only ones allowed to run the tea urn (and make an exorbitant profit).
It started with a couple of rugby socks chucked in the urn and worked its way up to full chemical warfare - the most potent solution being a mixture of butyric acid (the acid which gives stale sweat its rancid smell) and methyl isocyanide (a truly unique and disugusting stench).
Just a teaspoonful under the counter where the urn was parked was enough to bring the whole tea-making operation to a halt. In the end they had to call the caretaker in to scrub and repaint the entire room, but the smell lingered for a couple of months.
Of course, any offspring of mine contemplating such a course of action would deserve to feel the full majesty of the law!