Without this change it is not possible to use the MiniTest gem in a Rails app with Mocha in Ruby v1.8. There may even be problems just using the MiniTest gem in a Rails app in Ruby v1.8, irrespective of whether Mocha is used, although I haven't had a chance to investigate that any further. For a fuller explanation see the relevant [commit note](ce6395560c5434f1ab989c42acab1338fb8173f3).