Imagine delivering a fake NSL that's forged using a random case-number that happens to correspond to a real, active, and classified investigation, though. (Not even necessarily one that ever issued an NSL.) You'd ask them to talk to you about the case number, and then they'd completely clam up, because it is a case but it's one that doesn't involve you at all, so they don't want to admit of its existence to you. Kafka-esque results.