The Talmud (Shabbat 15b) details substances that, if stuck to a glass vessel, interfere (constitute a chatzitzah) with a successful immersion in a mikvah. Rashi (s.v. b’kli) comments that since glass is smooth, certain substances fall off on their own, and thus do not concern the owner (eino makpid) and thus they do not become a chatzitzah (see, however, Beit Yosef, Y.D. 202, citing the Smag). .

R. Shammai Kehat Gross, author of the Responsa Shevet HaLevi, questions this comment based on Rashi’s words elsewhere. In Massekhet Sukkah (6b) Rashi explains the notion of something that one is not makpid on not being a chatzitzah as a function of the fact that since the item will not be removed, it is considered part of the object it is attached to and thus not a chatzitzah. It emerges that the object is not a chatzitzah because it will remain where it is; while Rashi’s comments in Shabbat suggest the opposite, that the object is not a chatzitzah because it will not remain.

R. Natan Getstetner (Resp. L’Horot Natan, II, 72) explains that the key factor is actually the fact that the owner does not intend to remove the substance. Thus, the two reasons are one: since the substance will fall off on its own, the owner will have no need to remove it. Since the owner will not remove it, it becomes as if it is part of the object it is attached to (until it falls off).