Sometimes, when you're flossing, your gums might start to bleed a little. And for some people, the sight of blood makes them gag. The gag reflex could cause a brief episode of tachycardia. In a some cases this might trigger a bit of a panic attack, and if you're asthmatic, this could lead to a full-blown asthma attack.
So I guess the lesson for my patient receiving a neb treatment in bed 4 is clearly to stop flossing.