Marion's self-bondage session that ends with her wrists losing circulation and her hands turning black and developing blood clots. When Ally finds her and calls an ambulance, she deliberately leaves the ropes on so Marion doesn't do any more damage to herself. Marion spends the entire wait sobbing and begging Ally to untie her, and all Ally can do is grapple her so she cannot untie the ropes with her teeth in desperation.
And by "doing more damage to herself," we mean that if those ropes had come off and those clots had been allowed to enter her bloodstream, Marion could very well have died, either of a heart attack or a stroke depending on which part of her body the clots got to first.
What makes it even more traumatic for Ally is that Marion starts using her safeword in her pleas, under the panicked delusion that that will make her yield. Having to break the rule of the safeword in order to keep someone safe is pretty much any dom's worst nightmare.
Possibly veering into Fridge Horror and/or Black Comedy, Alan mentions a corset he created that was held on by the wearers body piercings. Lisa says it sounds cool and he agrees it was, until the wearer sneezed.