There is no tzaddik who has not committed an aveira. Likewise a ger who is sincere in the desire to be a frum Jew, may err, as do we all, but that does not take away from the geirus. BTW, one of the things that is said to a potential ger (my understanding, that is)is “Why do you want to be a Jew? You can be a wonderful non-Jew by following the 7 mitzvos of Bnei Noach and not be responsible if you don’t keep the mitzvos of the Torah. But the moment you are misgayeir, then you will be punished if you transgress.”
Logic dictates that if the above is said to a Ger-wannabe, that it means that once he or she is a Yid, it is for all time (again assuming the geirus was sincere l’chatchilah). FFB Jews commit aveiros and go OTD all the time, unfortunately. So might Geirim.