Bobcha, that is a halachic principle that applies to everyone (see, for example, Even HaEzer 4:30). I think that it is similar to a neder (in fact, the lashon of a neder is that something is prohibited to the noder like something the Tora prohibits). If he makes a clear mistake (taah b’devar Mishna) then it would be a mistake (similarly, it is possible to get out of a neder if one did not know a certain fact). I would think that it would only apply to the rav if he said that something is assur to you because it is generally assur (e.g. if he said that a certain food is not kosher) but not if he said that it is only assur to you or people in your situation (e.g. he ruled that a certain woman is a prohibited relative or you are a Cohen and he is a Yisrael and he ruled that she is prohibited to a Cohen).
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