On Wed, Dec 05, 2018 at 10:05:57PM +0000, Christopher Baines wrote:
> 
> substituting /gnu/store/mnvng3pdri5v8497j5a6nij53z3knlk1-ruby-sass-spec-3.5.4...
> downloading from https://mirror.hydra.gnu.org/guix/nar/gzip/mnvng3pdri5v8497j5a6nij53z3knlk1-ruby-sass-spec-3.5.4...
>  ruby-sass-spec-3.5.4  1.3MiB                                                                                                                       1.2MiB/s 00:01 [##################] 100.0%
> 
> sha256 hash mismatch for /gnu/store/mnvng3pdri5v8497j5a6nij53z3knlk1-ruby-sass-spec-3.5.4:
>   expected hash: 1kq3ndiv72krwb5h8bissdarm9lbbl7zbfbnzyfwnncsjh6p5xcr
>   actual hash:   0x0rzrvjm7q6x20bk5k2hc13zhp12njfga3b3azky4p2yxv60hcy
> substitution of /gnu/store/mnvng3pdri5v8497j5a6nij53z3knlk1-ruby-sass-spec-3.5.4 failed
> guix build: error: build failed: some substitutes for the outputs of derivation `/gnu/store/xizmf292d80dxgv35z2ik4zimrcfzrx5-ruby-sass-spec-3.5.4.drv' failed (usually happens due to networking issues); try `--fallback' to build derivation from source 

Are you still having this issue? It seems to work for me now.