While attempting to edit hiera yaml via the Horizon interface, I had an invalid extra space in my yaml. The error message given to me by Horizon when saving the form was:
Danger: There was an error submitting the form. Please try again. 11:51:27 and no hiera is saved
Which made me assume there was a bug with Horizon. It'd be nice to have some kind of useful 'invalid yaml syntax' error insteed.