If you're using should_not_allow_values_for and getting a failing test something in the lines of:
Failure:
test: User should not allow email to be set to "b lah". (UserTest)
[/usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.1/lib/shoulda/assertions.rb:56:in `assert_rejects'
/usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.1/lib/shoulda/active_record/macros.rb:174:in `__bind_