Mon Sotah 49: we'll explain Sotah 40

We ended up nobody should say anything while the Kohanim give the Bracha. Rather, they should concentrate on what they say.

Tosfos asks: we say thta if someone has a bad dream he should pray to make it good during Birchas Kohanim. However, how could he if he's obligated to be quiet and listen to the Brachos.

Tosfos answers: we allow him for perhaps there is danger. Sowe push off his obligation to listen in order that he can counter this possible danger. As we see, we even allow him to fast on Shabbos. So we see it's enough of a danger to push off Mitzvos for it.