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
https://web.mit.edu/sp255/www/reference_vault/Fatigue_Presentation.pdfBroken carabiner
https://www.osha.gov/doc/engineering/2014_r_05.html
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