This is now fixed. See github for details.
For a couple of weeks partial layouts have been broken in HAML. I opened an issue at Github.
The problem manifests itself by outputting the resulting HTML in the wrong place and sometimes twice.
The issue was caused by this commit, which attempted to "fix #capture when run with a block that returns a string." but broke partial layouts in the process. It did actually cause a test to fail, but it seems that was missed.
I've made a fork at https://github.com/ihid/haml which fixes this regression for now. You can update your Gemfile with:
gem 'haml', :git => "git://github.com/ihid/haml.git"