Yevamot 102B

ומי איכא כי האי גוונא? אין, דחזיוה רבנן לרב יהודה דנפק בחמשא זוזי מוקי לשוקא.

The Talmud asks: Is there really a case like this where people wear one shoe on top of another? The Talmud answers: Yes, for the rabbis saw Rav Yehuda, who went out once to the market wearing 5 pairs of shoes, which were similar to slippers, one on top of another.

אמר רב יהודה אמר רב: יבמה שהגדילה בין האחין ⁠— מותרת לינשא לאחד מן האחין, ואין חוששין שמא חלצה סנדל לאחד מהן. טעמא, דלא חזינן, הא חזינן ⁠— חיישינן.

Rav Yehuda said another halakha that Rav said: An underage yevama who grew up among her husband’s brothers before any ḥalitza was performed is permitted to marry one of the brothers through levirate marriage, and we are not concerned about the possibility that during the time she was in the company of her yevamin she removed a sandal from one of them, and thereby she would have already performed ḥalitza. The Talmud infers from this statement: The reason it is permitted to perform levirate marriage now is specifically that we did not see her remove one of their shoes, but if in fact we did see her do so, we are concerned and treat her as a yevama who already performed ḥalitza and is thereby forbidden to all the brothers.

והא תניא: בין שנתכוון הוא ולא נתכוונה היא, בין שנתכוונה היא ולא נתכוון הוא ⁠— חליצתה פסולה, עד שיתכוונו שניהם כאחד! הכי קאמר: אף על גב דחזינן ⁠— אין חוששין שמא כוונו.

The Talmud challenges: But isn’t it taught in a baraita: Whether he intended to perform ḥalitza and she did not intend to, or whether she intended to perform ḥalitza and he did not intend to, the ḥalitza is invalid, unless they both intended it as one to perform a proper act of ḥalitza? The Talmud answers: This is what Rav said: Even if we did see that she removed a shoe from one of them, we are not concerned that perhaps they intended to perform ḥalitza.