And what seems to be the explanation here, according to the words of the Chida (Devash L'fi Maarechet L, entry 12) who wrote the following: "Leil Shimurim (a night of watching) is for Hashem" - they asked Rabbi Chaim Vital (Rabbi Chaim Vital of blessed memory), because on the night of Pesach, a Jewish man climbed a ladder and fell and was injured, and how did this happen on a Leil Shimurim. And the Rabbi of blessed memory answered that the man committed a sin on that night, and because of that he fell and was injured. And from his words, it is clear that Leil Shimurim is when one engages in mitzvot, or at least when one does not go to commit a sin, but if God forbid one commits a sin, there is no protection. End quote.

The Chida added that this is also precisely from the language of the verse that says "Leil Shimurim hu l'Hashem" (a night of watching it is for Hashem), and it should have said "Leil Shimurim l'Hashem" (a night of watching for Hashem), but the word "hu" (it is) was added to emphasize that when it is a night of watching, it is when it is dedicated to Hashem. And this is as the Ramban (Nachmanides) explained earlier. Furthermore, the Chida deduced from the continuation of the verse "hu halayla hazeh l'Hashem" (it is this night for Hashem), that when it is a night of watching, it is when "it is this night for Hashem," meaning the night is sanctified to Heaven and no trace of sin is found in it, but otherwise, one's sin will catch up with him, because the blemish is greater on this night as we will explain with G-d's help.

And we learn from his words to resolve the contradiction in the matter — that all the sources indicating that this night is protected from harm speak about when it is dedicated to Heaven in recounting the Exodus from Egypt, in songs and praises, and in performing its mitzvot, and no spirit of sin has passed through it. But if, God forbid, a sin is committed on it, the protection is nullified, and for this reason, the Mekhilta said that one needs to be careful on it.