Examples in rspec-rails mix in Rails' assertion modules, which mix in assertion libraries from Minitest (if available) or Test::Unit. This makes it easy to accidentally override a method defined in one of those upstream modules in an example.
For example, if you have a model named Message, and you define a message
method (using def message or let(:message) in an example group, it will
override Minitest's message method, which is used internally by Minitest, and
is also a public API of Minitest. In this case, you would need to use a
different method name or work with instance variables instead of methods.