The spicy candy is a diabolical loyalty test imposed upon severed employees in the MDR department. It is heavily implied by Irving that someone failed at this test.
THEORY The spicy candy could have been put into the MDR vending machine disguised as a more traditional flavor, likely a popular one. When the hapless refiner took a bite of the taste surprise, expecting the burst of sweetness they craved, they instead encountered an intense burst of spices which innies are not often subjected to. The test: do they eat the candy, grateful for the bounty of Kier, or do they complain to the floor supervisor, showing themselves unwilling to trust that Lumon always has their best interest at heart?