Neville: As you see from RefoelZeev’s personal experience described above, though he was trying to point out that it is “only” a small percentage of BTs, he too personally knows 2-3 BTs who turned out to be gentiles *after* “becoming frum”. Who knows how many more of his acquaintance were also like that but he or even they never caught that they aren’t Jewish. And who can even count how many there are there out in the wider world.
And igorleningrad, above, brings another hencetofore undiscussed angle of this same problem. With the Russians. Again affecting large numbers of people.