Two carabiners chained together and then twisted can break. Has happened in lowering a stokes litter.

I have an REI carabiner from 1979 that has a pronounced crack running across the sharpest bend of the d shape. That guy is old enough to own one of those. There were a few Chouinard Biners from that era that were flawed and developed cracks around the gate pins.

MIT study Analysis of Fatigue Failure in D-shaped Carabiners

Broken carabiner