Jia and Liang (2017) showed that systems trained on SQuAD could be easily fooled by the insertion of distractor sentences that should not change the answer, and SQuAD 2.0 introduces questions that are designed to be unanswerable.
we argue that questions written to be unanswerable can be identified as such with little reasoning, in contrast to NQ's task of deciding whether a paragraph contains all of the evidence required to answer a real question.