Also, I think this should be an error, not a warning.  (Consider that calling `let` from `before(:all)` raises an error in rspec-core 3.0).  We should add a warning to 2.99, though.

And one last thing: I'm not sure that the suggestion to use `let` makes much sense (`let` is about defining a named dependent object; it's not about mocking, and also, `let` from `before(:all)` is not supported).  There are ways to get stub-like behavior in `before(:all)` that we could mention instead, such as using standard ruby constructs like defining singleton methods or extending a module onto a partial double.  I've done this before as a way of stubbing a method on an object that was only used in `before(:all)`.  That also might be confusing, though...given that this is kinda a complex issue that's hard to summarize in an error message it might be good to have it refer to a gist or wiki entry or something that explains the issue and available work arounds.