Finally figured out the right thing to Google: "scss stack level too deep". Turns out it's a sass issue per this SO: "rake aborted! stack level too deep" while deploying to Heroku
TL;DR:
I had this in my Gemfile:
gem 'sass-rails', '~> 3.1.4'
Needed this:
gem 'sass-rails', '3.1.4'