Rails gotchas: ruby-enhanced yaml != rails-enhanced yaml

I you've just added some funky ruby-generated something into your yaml fixture and are suddenly getting this error message:

Fixture::FormatError: a YAML error occurred parsing /myproject/config/../test/fixtures/widgets.yml. Please note that YAML must be consistently indented using spaces. Tabs are not allowed. Please have a look at http://www.yaml.org/faq.html
The exact error was:
  SyntaxError: compile error
(erb):8: syntax error, unexpected ')'
_erbout.concat "  created_at: "; _erbout.concat(( '1 January 2006'.to_date.to_s(:db) -).to_s); _erbout.concat "\n"
                                                                                       ^

Check to make sure you haven't done the "good" thing and used -%>. It appears that YAML files don't like the whitespace-eating ending... you can only use: %>