Let's step back and make sure I understand the comment in the code.  I believe it is referring to a set of bookkeeping that needs to happen (if I understand it correctly).  Either the thing that calls the callback needs to keep track of what's been called and make sure that things aren't called "out of order" or the callback needs to be resilient to potentially getting called "out of order."  Is that true, or is it talking about something else?