One who accidently switched an item with another person:
- If one attended an event, or participated in a Minyan or Shiur, and at the end of the event he accidently took the wrong coat, hat, umbrella, or Tallis/Tefillin or any other item, may he use that item until he finds the rightful owner?
- If one attended an event, or participated in a Minyan or Shiur, and at the end of the event he noticed that someone accidently took his coat, hat, umbrella, or Tallis/Tefillin or any other item, may he take that persons item and use it until he finds the rightful owner and makes the switch with him?
One who [accidently] took the wrong item while at the house of a mourner or while participating in a festive meal, may not use the item [that he accidently took] and doing so is considered stealing. The same applies if one’s item was accidently taken and he then desires to take the item of the person who took his item, he may not use the item and doing so is considered stealing. This prohibition applies even if one knows that the other person is [stealing and] using his item [without his permission], he may nevertheless not make use of his friends item [unless he receives explicit permission from him].
Returning the item: Upon the rightful owner of the item being found the person must return him the item even if his own item became lost [not due to the fault of the rightful owner].
Laundry mat mistake: If a gentile [or Jewish] laundry mat which gave one the wrong shirt, it is forbidden for him to use it, and he must return it to the rightful owner even if his own shirt became lost.
What is the law if many days passed and one has still not found the rightful owner?
Some Poskim rule that if one knows that the rightful owner has given up hope in finding the item [Yiush] then he may use it. If many days passed and one has not seen or heard from anyone looking for his item, then one can assume that he has given up hope in retrieving it. Other Poskim however rule that one may not use the item even after many days have passed [and must hold on to it until Eliyahu Hanavi points out its rightful owner].
What is the law in a case that it is very common for the items to be switched and people are not particular in this matter?
In any case that the custom of the world is not to be careful against having their item switched, and people constantly take their item while they constantly take other people’s items, then one may use the item that he mistakenly took. This commonly occurred in previous times with regards to shoes or slippers, that people would remove their shoes prior to entering the building and the shoes would become mixed up and each person would take whatever pair he finds. [Although such a situation today is not common anymore, in the event that there is an item that is commonly switched the above allowance is applicable.]
What is one to do if he switched Talleisim or Tefillin with another person?
One may use the Tallis [or Tefillin] as if it were his until he is able to switch it back with the person who took his Tallis [or Tefillin]. Upon being asked to switch back one must do so.
 Admur Gzeila Ugineiva 30; Michaber C.M.136/2; Baba Basra 46a
 The reason: As even if in truth his friend is using his item, if he were to use his friends item in exchange [without receiving his explicit permission] it is considered as if he is stealing from a stealer, and it is forbidden for one to place judgment alone without a Beis Din, as was explained above in Halacha 27. [Admur ibid; See however Aruch Hashulchan 136/2 who seems to learn that if one knows that the owner is using his item then he may use the owners item even without his consent. Admur ibid clearly negates this approach.]
 Admur ibid; Rama ibid; Terumos Hadeshen 319
 Admur ibid; Rama ibid; Terumos Hadeshen 319
 Taz C.M. 136 based on Mordechai regarding a laundromat; Nesivos Hamishpat 136/2 records the opinion of Taz and explains in Mishpat Haurim 136/2 that perhaps even the Taz was only lenient in the case of a laundromat being that if the person did not give up hope he would have heard about it from the laundromat; See Kesef Kedoshim for various answers on the questions of the Ketzos Hachoshen against the Taz.
 The reason: As the item has legally become his due to Yiush and Shuiy Reshus. [See Taz ibid]
 Ketzos Hachoshen 136/2; Pischeiy Teshuvah 136/1; Nesivos Hamishpat in Mishpat Haurim 136/2 limits the allowance of the Taz ibid to specific , as explained in the previous footnote
 The reason: As even if the owner did give up hope in finding it, nevertheless, it does not help for one to give up hope after the item has already reached one’s possession [Ketzos Hachoshen ibid], as brought in Admur Hilchos Metzia Upikadon 2. Furthermore, it does not even help to be allowed to use the item with intent to return when the rightful owner is found, as explained in the second opinion in Admur Halacha 2, and so is the main ruling. See however Kesef Kedoshim ibid for various answers on behalf of the Taz and why the above case is different than a case of a found object.
 Aruch Hashulchan 126/2; Kesef Kedoshim 126
 Birchas Habayis 37/10; See Kesef Kedoshim 126
 The reason: Although we rule that when one switched an article with another it is forbidden to use it due to stealing [Admur Gzeila Ugineiva 30; Michaber C.M.136/2] nevertheless by a Tallis since we assume each person does not mind if the other uses his Tallis until they switch it back, as even in a case that the Talleisim were not switched one may use his friends Tallis without permission under this basis, and thus certainly in this case it is permitted. [ibid]