A: See the Gemara in Pesachim 8b. "Harm will not befall one on the way to perform a Mitzvah" is said in regard to an infrequent danger. The example given there is one who is checking for Chametz.Should he check under rocks? No, he should not, because there may be snakes or scorpions under the rocks. The Gemara asks – how so?Harm will not befall one on the way to perform a Mitzvah. Answer: This (snakes and scorpions under rocks) is a frequent occurrence and there is therefore a chance that he may be harmed. Another example: A person has a joint wall with a non-Jewish neighbor.Perhaps he should stick his fingers into the cracks in the wall to search for Chametz? No, he should not check because the neighbor may accuse him of witchcraft and cause him terrible problems. The Gemara asks – how so?Harm will not befall one on the way to perform a Mitzvah. Answer: This is an evil neighbor and it is considered a frequent occurrence. We do not apply this principle when there is a frequent occurrence. The Gemara states that the proof for this idea is found in the Book of Shmuel (1 chap. 16) when Hashem tells the prophet Shmuel to anoint David as king."And Shmuel said: How can I go - if Shaul hears he will kill me?"(verse 2).Hashem said: Tell him that you are going to offer a sacrifice in order that he will not be suspicious.The Gemara asks: But he was going to perform a Mitzvah directly told to him by Hashem and "Harm will not befall one on the way to perform a Mitzvah" (see Rashi)?Answer: When there is a frequent danger, even those performing a Mitzvah can be harmed.This is discussed at length in the book "Mesilat Yesharim" at the end of chapter 9. It is obvious that if there are terrorists living among us, it is considered a frequent occurrence and even people performing a Mitzvah can be harmed.