Stealing by Accident?

Question: If one accidentally took and used a friend’s similar coat, is he considered a ganav (thief)? Is he obligated to pay kefel (double)? Must he pay the owner if something happens to it (onsin)? [The querier then presented sources he found about geneiva b’shogeg (unintentional theft).] How can there be geneiva b’shogeg considering one needs intention to acquire something?

Answer: We will only scratch the surface of the scholarship on whether one is obligated for geneiva b’shogeg and relate to some of the issues you raise.
The K’tzot Hachoshen (25:1) is among those who posit that a ganav b’shogeg is exempt. He infers this from Rishonim, but his main rationale is that the concept of culpability for accidental financial harm to his friend is limited to mazik (one who physically damages another’s property) because it specifically is derived from a pasuk. Thus, if one takes another’s object without damaging it, he is not responsible to pay for it. Of course, he has to return it when he finds out the truth, but the matter is important if it was lost, damaged, or passed on to someone else.
The Machaneh Ephrayim (Geneiva 7) cogently presents opinions of Rishonim, but agrees with those who obligate a ganav b’shogeg. He is particularly impressed by the gemara (Pesachim 32a) concerning payment made by one who accidentally ate teruma, which says that if the food’s price went down after he ate it, he pays the higher price because "it is no less than one who steals." The Machaneh Ephrayim sees this as proof that there is payment for geneiva b’shogeg.
Let us now discuss your quandary about the need for intention. The gemara (see Bava Kama 79a) does speak of a kinyan (an act of acquisition) as a necessary step for the obligations of a ganav, and kinyanim require a certain level of intent. However, not all of the levels of intent pertinent to geneiva are equal to those regarding other acquisitions. If one lifted up an object to move it out of his way, he would clearly neither acquire nor be considered stealing it. If he wanted to use it without ever returning it, this would be intention for theft even if he tried to be "shrewd" by having in mind to "not acquire it" (it indeed would not become his). Furthermore, even one who intended to briefly borrow something without permission is considered a ganav (Shulchan Aruch, Choshen Mishpat 359:5).
The Machaneh Ephrayim makes a relevant fundamental distinction. Geneiva b’shogeg can be culpable when one intended to bring the object from another’s possession into his own. If one thought he was just continuing using his own, that would not be considered an act of stealing. (This idea is indeed parallel to the halacha regarding intention to acquire something legally (see Yevamot 52a).) According to this, the accidental coat switcher is not even a ganav b’shogeg and does not have, as of the time he took the object, the accompanying responsibilities for its welfare. Cases in which geneiva b’shogeg applies include unknowingly buying a stolen object or even borrowing one.
The Marcheshet (II:32) posits that a ganav b’shogeg has the basic obligations of a ganav. He sees the K’tzot Hachoshen’s source to exempt – the obligation of an unintentional mazik – as the source to obligate an unintentional ganav as well. As such, though, just as a mazik is exempt b’oness (under extenuating circumstances), so too a ganav b’oness is exempt. In our case, taking another’s coat is usually shogeg rather than oness. According to this approach, it could be considered geneiva. Regarding intention, he does intend to use something that turned out to actually belong to someone else.
All agree that one is not disqualified for anything (e.g., testimony) due to such an unintentional aveira. Kefel is never levied in our days, and it is thus not discussed much by poskim. However, logic and implicit statements indicate that this k’nas (penalty), which applies to only certain types of theft and when one is exposed by witnesses, is predicated on full culpability and does not apply b’shogeg.