Re: moving the goal post
Andrew, imagine that mastering a technique is like building a structure (bear with me here). You do it with a partner and you both have a limited knowledge hot to go about it. At this point of structure building career any structure you finish is full of imperfections and structural weaknesses.
Now imagine that from time to time your partner sees an imperfection in your "work in progress" leans on it and breaks it down. Is this a good thing? It depends really, as part of your learning process you need, occasionally, to to finish your structure, warts and all, just so you can learn. Also as part of your learning process, from time to time, you need to be shown those weaknesses, otherwise you will have an unrealistic expectations of your work.
The challenge is to get this balance right and this is mainly the responsibility of your teacher (the master builder) but also yours and your partner's.