Original story from Mishpacha in Hebrew here).
A certain Ba’al Teshuva who had been learning in Israel for two years was eating at a kollel couple’s house on Shabbos. During the meal, it became apparent that the Ba’al Teshuva did not like cholent. The kollel husband remembered reading that one who does not eat “chamin” on Shabbos is to be suspected of being an min (heretic). In addition, he also realized that the Ba’al Teshuva did not shukkle (sway rhythmically back and forth) when he davened (prayed), as many Orthodox Jews do. Upon observing these two behaviors and realizing that the Ba’al Teshuva comes from a country where Judaism is not kept very well, he reasoned that the Ba’al Teshuva must not be a Jew according to halacha. Further questioning of the Ba’al Teshuva’s background turned up nothing to indicate that he was not Jewish. Nonetheless, the question was forwarded to Rav Eliyashiv, who determined that the man must undergo a giur (conversion).