MRT fails to write to one or more output textures - cause and solution

I had a strange issue when I was trying to write to multiple textures in a single shader pass (MRT), that having activated several targets (glDrawBuffers), only the first one was getting any output, the others were not being changed, other than being cleared correctly to my desired clearcolor.

The problem turned out to be a combination of two things.
Firstly, I had Blending enabled, which meant that my current blendfunc would be applied to output pixels, and secondly, my fragshader was outputting RGB vec3 values, leaving alpha unaltered.

If you're playing around with MRT, and one or more outputs refuse to get written to, doublecheck that you have blending disabled, and/or check your pixel alpha.