See this Comm Center thread by Larry Groom (and others in the past!). Need ACP Help on how to recover from authoring failures which render the content unusable. This should also be in the Authoring System item as a separate heading "Recovering from Editing Mistakes"
The info is both in the web authoring info as well as in ACP Help where I can reference it from tickets on the Comm Center.However it is incorrect. The latest saved one has the errors in it (it is saved after editing). Correct this.