משנה אויף אסתר 4:4

If one makes a hollow in a water-spout to collect pebbles, its water disqualifies the mikveh; In the case of a wooden [spout] if it holds even a little, But in the case of an earthenware [spout] if it will hold a quarter-log. Rabbi Yose says: also in the case of an earthenware [spout] if it holds even a little: they have spoken of "a quarter-log" only in the case of broken sherds of an earthenware utensil. If the pieces of gravel moved about inside [the hollow], it disqualifies the mikveh. If dirt went down into it and was pressed down, [the mikveh continues to be] valid. If the spout was narrow at each end and wide in the middle, it does not disqualify [the mikveh] invalid, because it had not been made to gather anything in it.

A stopper of a jar that is loose but does not fall out: Rabbi Judah says: it protects. But the sages say: it does not protect. If its finger-hold was sunk within the jar and a sheretz was in it, the jar becomes unclean. If the sheretz was in the jar, any food in it, become unclean.