So it looks like MacOS Sierra broke pbcopy/pbpaste for tmux. This manifests as clipboard errors when using netrw/vinegar in neovim under tmux. (Dunno why netrw is writing to the clipboard but anyways).

The solution is to use Homebrew to reinstall a patched version of reattach-to-user-namespace:

brew uninstall reattach-to-user-namespace
brew install reattach-to-user-namespace --with-wrap-pbcopy-and-pbpaste

Although this was annoying, it did lead to running brew cleanup which reclaimed 3.8GB of disk space. Jinkies!