It was stated: A side-post put up accidentally [not for the intention of Shabbat], Abaye ruled, is a valid side-post, but Raba ruled: It is no valid side-post.
They differ only where [the residents] did rely upon it on the previous day.
That is, if they used it on the day before Shabbat then it would count. For example a proper side-post which fell down on Shabbat. If we relied on an “accidental” side-post – some other item – it would count.
These side-posts can be made of anything, by the way. According to the Mishnah: even living animals! (I’m envisioning a cow tethered inside an alley. “Stay, Bessie!”) The same holds true for documents – including a writ of divorce! (now that has to be a scene in someone’s movie: a woman receives her divorce written on a cow’s hide. Hilarity ensues.)
But it is another scene found on this page which I find interesting:
Come and hear [of the incident] where Rab was sitting in a certain alley and [his student] R. Huna sat before him when [Rab] said to his attendant, ‘Go, bring me a jar of water’. By the time the latter returned, the side-post fell down and he motioned to him with his hand to remain in his place.
Said R. Huna to him, ‘Is not the Master of the opinion that one may rely upon the palm-tree?’(which happened to be at the entrance)
‘This young Rabbi’, he replied: ‘seems to think that people cannot explain a ruling they have heard! Did we rely upon it since yesterday?’