> I wasn't planning that so much for the adaptor, as it in itself is about converting the interface.

You made comments somewhere (I forget which PR or email) about the fact that you plan to refactor the adapter significantly, and given that, it worries me to only have tests covering it with mock expectations.  What do you think about pulling one of the built-in formatters from rspec 2.14 and making it the test fixture (renamed appropriately).  Then, as you refactor, you can assert that the output remains the same.  While that doesn't prove that you've properly adapted to all possible legacy formatters, it would go a long way towards showing that it preserves the same output for a "representative" legacy formatter (which will hopefully use all or most of the events).

This idea is really a bit like a "golden master":

https://github.com/kytrinyx/approvals