If you guys want the RAW answer:
5e DMG pg 235 "Rolling behind the screen lets you fudge the results if you want to If two critical hits in a row would kill a character, you could change the second critical hit into a normal hit, or even a miss, Dont distort die rolls too often, though, and don't let on that your doing it. Otherwise your players might think they dont face any real risks-- or worse, that you're playing favorites."
And again on page 237 "Remember that dice don't run your game - you do. Dice are like rules. They're tools to help keep the action moving."
So according to 5E RAW, DMs are expressly directed to secretly ignore dice results, to keep the action moving and protect the PCs (and the Story) from bad results through no fault of their own.