Two at minus-twenty-something F.
First: I was twenty-something, with GF>wife, went in Miller Meadows above Tahoe West shore, mild day, NF dbl-wall pup-tent, 15 degree Snow Lion SuperLite down bag, on--blue foam, cause too poor for ensolite! Slept on stomach, shivering up enough warmth to fall asleep; shiver; repeat. Early hours, began hearing large-caliber rifle fire all around us. Got home & mutual friend said, "whoa, it was twenty below at Tahoe this weekend."
Expect expansion of water in dead tree limbs produces rifle sounds.
Second: Marcy Dam in Adirondacks, with 13yo nephew/godson on our first outing. Above freezing when we got NNN-BC gear at Adirondack Loj. Eschewed lean-to for Megamid. Two Marmot goretex (I know) Snowgoose 0deg down bags on thermarests on ridgerests. Massive ice plate on bag by chin; only ever frost before. Rental boots solid ice. Twenty minutes jumping-jacke; fire stove & feed nephew; get everything together while P still in bag; say, "Now!" Balmy zero back at the Loj, where they said it was twenty below last night. To cheat death we had brushed out teeth with Skittles.