I did this once. Lost Mine of Phandelver, the dragon encounter.
I did a pretty dramatic bit of roleplay with the dragon beforehand where he tried to bunch them all up before opening with his breath attack.
They were level 3. They never stood a chance. I read out the number and it would have been an insta-death (no death saves) for multiple members of the party, with everyone else unconcious. Luckily, they didn't hear me well, and when asked to repeat it I lowered the damage considerably. No insta-deaths, one stayed conscious, along with the powerful NPC they had recruited to help.