A person who wears a four-cornered garment without tzitzis transgresses a mitzvah min haTorah to wear tzitzis. If a person is among a large group of people and notices that his tallis katon has become invalid, he is not obligated to remove his clothing immediately to take off his tzitzis. He should instead make his way to a private area and remove his tallis katon there. Since it would be very embarrassing to disrobe in public, he is permitted to transgress the mitzvah min haTorah in this passive way. In contrast, removing a tallis gadol in public is not embarrassing, and he must take it off immediately if it becomes invalid. The poskim disagree about whether removing a tallis gadol in shul is considered an embarrassment. If one finds himself in this situation, he should render his tallis ownerless to absolve himself of his obligation of tzitzis.
[שו"ע יג, ג, ומשנ"ב טו ו־טז; ביאורים ומוספים דרשו, 20 ;וראה שם, 22]