As in Oh-shit moment I was thinking more of tank specific things. Taunt swapping issues/other tank dying/getting bonds when you weren't the tank who was meant to.
But tbh, I've played healer, if the healer can't throw the external on you despite shit hitting the fan (Especially considering it's off the GCD, I mean come on) then they're a really shitty healer. Just make a macro for each tank, if the tank calls it, hit the macro. Shit isn't on the gcd so you can literally doing it while waiting the gcd to cast your next ability.