Shabbos Daf 9 שבת דַף 9

Rav Yitzchak bar Avdimi said that R’ Meir was of the opinion that it is forbidden to use a post ten tefachim high and four tefachim wide in a reshus hayachid to rearrange a burden on, even though the post has the status of a reshus hayachid and the person is standing in a reshus hayachid as a safeguard against doing the same on an elevation in a reshus harabim, which would then involve a violation of transferring from a reshus harabim to a reshus hayachid. 

Rav Acha bar Yaakov explains the various reasons why the Mishna forbids beginning certain activities on any weekday shortly before minchah gedolah until he davens as precautionary measures that one should not come to miss Minchah:

Haircut – Lest the barber’s scissors break.
Bathhouse – Lest someone going to use the steam bath becomes faint.
Tannery – Lest someone going to inspect his hides discovers a flaw in his merchandise and becomes preoccupied because of it.
A small meal – Lest he extends his meal.
Judge a case – Lest a judge at the end of the case dealing with his verdict sees a reason to reconsider and overturn his verdict.

The Mishna stated that when one has already begun one of these activities, he does not need to interrupt them in order to daven. The Gemara defines the beginnings for each of the activities:

Haircut – When one puts the barber’s cloth on his lap.
Bathhouse – When one removes his clothing.
Tannery – When one slips his arms out of the sleeves and ties the sleeves behind his back.
Meal – Rav said when one washes his hands. R’ Chanina says when one unfastens his belts. The Gemara clarifies that R’ Chanina was referring to those in Bavel who were accustomed to gird themselves tightly.