Why use class variables here?  class variables are one of those ruby constructs that are generally best avoided.  On top of that, within the RSpec DSL it's not terribly clear what the scope of a class var would be given it's not a normal `class Foo; end` class definition.

Instead, just use an ivar.

Also, I'd just set it to a scalar value (e.g. `@filtered_before_all_invoked = true`).  There's no need for the added complexity of making it an array of one element, given nothing else adds to the array.