Kilayim, Chapter 9, Mishnah 5. This mishnah deals with clothing merchants and the question of how they may go about their trade and display garments of sha'atnez to their customers.
The text of the mishnah:
"Mochrei kesus mochrin k'darkan" - clothing merchants may sell even garments of sha'atnez in the ordinary manner in which they were accustomed to sell. The ordinary manner was to place the garments on their shoulders and spread them out so that customers could see the merchandise; not folded garments only, but spread out and displayed to the eye. Obviously the sale itself was made to non-Jews, but the merchant needed to display the garment. This was permitted because he does not intend to wear the garment, only to show it.
An unintended act:
From here we see that an unintended act - where a person performs an action with no intention of wearing the garment, but only of showing it off and displaying it - is permitted. This is not the opinion we saw earlier, that an unintended act is forbidden, but rather the opinion that holds an unintended act is permitted.
The comment of Rabbi Akiva Eiger:
Rabbi Akiva Eiger comments that the mishnah must be understood as referring to a case that is not a p'sik reisha - that is, where it is not certain that the garment will be considered as being worn by him. The permission was stated in a case where the garment might ultimately end up upon him, but this is not inevitable. For if it were done in a manner in which he actually wears the garment, there would be a problem here regardless of his intention and his thoughts.
"U'vilvad shelo yiskavein bachamah mipnei hachamah u'vageshamim mipnei hageshamim":
The permission is limited to using the garment for the purpose of display alone:
"Bachamah mipnei hachamah" - he may not intend that the garment shield him from the sun.
"U'vageshamim mipnei hageshamim" - and likewise he may not intend that it shield him from the rain.
The reason for this: with such an intention the garment is fulfilling its function as a garment and as an article of clothing, and the problem therefore returns.
The practice of the scrupulous:
Although according to the first tanna clothing merchants may place the garment upon themselves and on their shoulders in order to display it, the mishnah concludes: "Hatzenu'im mafshilin b'makel" - those who are meticulous in mitzvos would hang the garment on a stick and not place it on their bodies, out of concern that they would ultimately come to transgress the prohibition of sha'atnez.
In summary: in this mishnah we learned that clothing merchants sell in their ordinary manner and display even garments of sha'atnez on their shoulders, since their intention is display and not wearing - and from here we see that an unintended act is permitted, provided that it is not a p'sik reisha, as Rabbi Akiva Eiger comments. The permission is conditional upon his not intending to shield himself from the sun or from the rain, for then the garment resumes its function as clothing; and beyond the letter of the law, the scrupulous were accustomed to hang the garment on a stick rather than place it on their bodies.