Rif Shabbat 27b

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27b

{Shabbat 60a continues}
and they [some fugitives] were hiding in a cave. They proclaimed, 'He who would enter, let him enter, but he who would go out, let him not go out.' {For fear of spies, lest their whereabouts be disclosed.}
Now, the sandal of one of them became reversed, so that they thought that one of them had gone out and been seen by the enemies, who would now fall upon them. Thereupon they pressed against each other {in panic}, and they killed of each other more than their enemies slew of them.
Rabbi Eleazar ben Elisha {our gemara: Rabbi Ilai ben Eleazar} said: They were stationed in a cave when they heard a sound [proceeding] from above the cave. Thinking that the enemy was coming upon them, they pressed against each other and slew amongst themselves more than the enemy had slain of them.
Rami bar Yechezkel said: They were stationed in a Synagogue, when they heard a sound from behind the synagogue. They thought that one of them had left. {preceding sentence an accidental copy from the first cave story.} Thinking that the enemy was coming upon them, etc. {The gemara continues: they pressed against each other and slew amongst themselves more than the enemy had slain of them.}

In that hour it was enacted: A man must not go out with a nail-studded sandal. And it does not matter whether it is on Shabbat or on Yom Tov {he may not go out}.

{Shabbat 60b}
What is the reason on Shabbat, not?
Because it is day of assembly when there is an interdict against work {just as when the incident happened.}
So too on Yom Tov, it is day of assembly when there is an interdict against work.

And thus we learn {tnan - in Betza 14b}: But one may not [send] a nail-studded sandal or an unsewn shoe [on Yom Tov].

Rav Yehuda cited Shmuel: They learnt this only [where the nails are] to strengthen [the sandal], but where they are ornamental, it is permitted.

And how many [nails] constitute an ornament?
Rabbi Yochanan said: Five on each.
And Rabbi Chanina said: Seven on each.

{see the gemara for a different girsa}
Rabbi Yochanan said to Rav Shemen bar Abba: I will explain to you: According to me, two on this side and two on that side, and one in the strapping. According to Rabbi Chanina, three on this side and three on that side, and one in the strapping.

Ilfa {our gemara: Efa} said to Rabba bar bar Chana: You, as disciples of Rabbi Yochanan, should act as Rabbi Yochanan; but we will act as Rabbi Chanina.

A certain shoe-maker asked Rabbi Ammi: What if it is sewn from within?
{Rashi: i.e., a leather shoe was placed inside a sandal and sewn thereto.}
He said to him: It is permitted, replied he, but I do not know what is the reason. {I.e., he had heard this ruling, but did not know why.}
Rav Assi {our gemara: Ashi} said: And does not the Master know what is the reason? Since it was sewn from within, it becomes a shoe. The Rabbis enacted a decree in respect to a sandal, but in respect of a shoe they did not enact any decree.

Rav Abba bar Zavda asked Rav Abba bar Avina: What if he arranged them [the nails] zigzag-shape? {Kalbus is a tongs or pinchers, which presumably opened X-wise}
He said to him: It is permitted.
It was stated {by Amoraim} likewise:
Rabbi Yossi bar Chanina said: If they are arranged zigzag-shape, it is permitted.

Rav Sheshet said: If the whole of it [the sole] is covered with nails [underneath] so that the ground should not wear it away. it is permitted.
They learnt {in a brayta} in accordance with Rav Sheshet: A man may not go out wearing a nail-studded sandal, nor may he stroll [in it] from house to house, and even from bed to bed. But it may be handled in order to cover a utensil or support the legs of a bed therewith; but Rabbi Eleazar the son of Rabbi Shimon forbids this. If most of its nails are fallen out, but four or five are left, it is permitted; while Rabbi Yehuda permits it up to seven. If one covers it with leather underneath and drives nails into it on top, it is permitted. If one arranges them [the nails] zigzag-fashion, or flattens [them] out, or points [them], or covers the whole of it with nails so that the ground should not wear it out, it is permitted.

"NOR WITH A SINGLE [SANDAL], IF HE HAS NO WOUND [or, BRUISE] ON HIS FOOT":

{Shabbat 61a}
The reason is that he has no wound on his food.
Hence if he has a wound on his foot, he may go out - With that [worn] where there is no wound - like Chiyya bar Rav, and like Rabbi Yochanan, who said {regarding the fact that he was given only his right sandal}: You treat the left foot as though it had a wound!

"NOR WITH TEFILLIN, NOR WITH AN AMULET, {IF IT IS NOT FROM AN EXPERT}":
Rav Papa said: Do not think that both the man [issuing it] and the amulet must be approved; but as long as the man is approved, even if the amulet is not approved.
This too may be proved from the wording of the Mishna, for it states, NOR WITH AN AMULET WHEN IT IS NOT FROM {min} AN EXPERT, and does not say "WHEN IT IS NOT EXPERT {=approved. In fact, the girsa in the printed text of our Mishna in the Rif lacks the word min, while the girsa in the printed text of the Mishna in our gemara has the word min.}
We indeed do deduce this.

The Sages learnt {in a brayta}: What is an approved amulet? One that has healed [once], a second time and a third time.