This is called spontaneous recovery: broken associations are rapidly suppressed but not, in fact, unlearned; given enough time, they reemerge. Further, if after a long stretch of trials with a broken association (buzzer but no food), you reinstate the association (sound a buzzer and provide food again), the old association will be relearned far more rapidly than the first time the dog experienced the association between the buzzer and food. This is called reacquisition: old extinguished associations are reacquired faster than entirely new associations.
