The Constraints of Cutting Across a Crowded Shul
It is forbidden to pass within daled amos (four arm-lengths, a distance totaling about six feet) in front of a person davening (praying) Shemoneh Esrei (the silent Amidah, the most serious of the prayers); the basic reason for this is so as not to disturb the davener’s kavanah (concentration). Passing on the person’s side is permissible—as long as the passerby is not visible to the one davening. According to the Zohar, however, the restriction applies to an entire four-amah radius around the davener—back and sides included. According to some opinions, these bounds also apply when passing someone who is reciting Kaddish or Shema.
The same constraints apply when taking the three steps back at the conclusion of Shemoneh Esrei and stepping backward would encroach on the daled amos of the davener behind (regardless of when the latter started Shemoneh Esrei).
Poskim decry the lapse in knowledge and care afforded this halachah in many shuls. Several ways of limud zechus (viewing [a lapse] favorably) have been marshaled to justify this breach: If the davener is situated in a passage-way—which is not an acceptable place for davening—it might be permissible to pass. Even if the davener is standing in a proper place there is a possibility to pass under the following conditions: a substantial barrier, such as a shtender (prayer lectern) or a table, stands between the one wishing to pass and the one davening; the davener’s eyes are closed; or their siddur (prayer book) is blocking their view of passerby.
We should be careful to observe this halachah even when these “passes” are available. However, at times a congregant has to leave their makom kavua (designated place for prayer) in the midst of davening, such as for a mitzvah overes (time-sensitive mitzvah) —or, especially, if in need of the facilities; under these circumstances, there is room to rely on the above-mentioned heterim (leniencies).
(See Halachah #186 regarding sitting idly within daled amos of a person davening Shemoneh Esrei.)