How do I get a full stack trace instead of "Unexpected error while processing request" in Rails test environment?

When starting my rails server in test environment (rails s -e test), something is rescuing exceptions and outputs instead "Unexpected error while processing request: ...error here...". I get the backtrace in development mode.

Who is it, and how can I disable that? I need the full backtrace, not just the error that was raised!

I'm using Rails 3.2.3, Ruby 1.9.3, rspec, capybara, capybara-webkit