A scary looking error with an incredibly simple solution.
The full error in my case was:
Error: Node Sass does not yet support your current environment: OS X 64-bit with Unsupported runtime
I intermittently see this error when I try to run a build of a project I haven't worked on in a while, and in the intervening period I've updated my global installation of Node. It's one of those ones that's always faintly alarming looking to begin with, until you remember it's a one liner to fix.
The solution? Run this command in the affected project:
npm rebuild node-sass
It'll run for a couple of minutes, and bob's your uncle - you should be able to run your tasks again.