Often times in the course of halachic discussions it will be related that a certain Rav did such and such. When do such accounts have halachic significance? How do we safeguard against inaccurate/urban legend stories detrimentally?
One part of it, I think, is that the Rav himself has to make sure that his actions will not be misconstrued by observers. We thus find, for example, cases where our Sages acted according to the strict letter of the law (or it was suggested that they do so), even though they might have preferred to show deference to a different view, "lest the students see it and [wrongly] decide the halachah accordingly for future generations" (Berachos 11a, Pesachim 100a, and Kesubos 50b).
The first person recording the Ma'aseh Rav is a talmid of the Rav, and it generally has to be in writing by a well-published talmid of that Rav. For the most part, if we're using a Ma'aseh rav to decide halacha after more than a single generation, that Rav is somebody of extreme importance, like the Ari Z"l or the GR"A. (Or, if you're chassidish, the Rebbe of your branch of chassidus.)