the practice of "sitting shiva" for someone who leaves orthodox practice is bsed on a typo
Recent scholarship has shown that the source of the original custom, a story published in the twelfth century by Rabbi Isaac ben Moses of Vienna in Or Zarua regarding Rabbi Gershom ben Judah, contained a typo and was thus misunderstood. Rabbi Gershom had a son who had converted to Christianity. A text that had been read as, "Rabbi Gershom sat shiva for his son when he converted [Heb. k'she-nishtamed]", turned out to have been "Rabbi Gershom sat shiva for his son who had converted [Heb. she-nishtamed]", when the son actually died years later of natural causes.