This is really just a heads-up for anyone encountering issues with Octoprint + smoothie.
If Octoprint is set to issue 'M21' (initialise SD card) smoothie responds with "!!" and Octoprint flags an error and terminates coms. Setting octoprint to ignore error messages doesn't fix it, as smoothie responds to subsequent 'G' codes with "!!" too (this looks like a bug). Weirdly 'M' codes still seem to work.
Anyway, the simple fix is to uncheck the 'Initialise SD card' option in the Octoprint settings, as its not needed.
