The last favor should be one you can give freely, not one that keeps the ending uncertain.

Endings often come with one last request, and the request can do two different jobs. Given freely, it closes something warmly and both people walk away clear. Given as a thread left deliberately attached, it keeps the door ajar and keeps everyone waiting for a next chapter that is not coming.

If you are near the end of something, look at the favour on the table and ask whether you could do it and be done. If yes, do it well. If it would only keep the question open, say so gently and let the ending be one.