The fifth chapter of Shabas in the Bavli discusses various things an animal can wear in the street on Shabas. Some of them are forbidden because the item may fall off the animal and a person may come to carry the item around in the street, a prohibited activity. The Talmud itself offers this reason; other times, Rashi does so. Some examples of the latter:
- On daf 53 amud 1, the Talmud is discussing a sort of horseshoe: a horse cannot go out wearing it. Rashi explains "דדילמא מישתליף ואתי לאתויי", "because it may come off and he will come to bring [it]".
- On 53 amud 2, the Talmud is discussing a receptacle placed under an animal's udders: it says that the animal cannot go out wearing that receptacle if it's loose. If it's tight, though, it's okay, as Rashi explains, "דליכא למיחש דילמא נפיל ואתי לאתויי", "because one [need] not be concerned lest it fall and he will come to bring [it]".
- On 54 amud 1, the Mishnah is discussing a saddlecloth or blanket for an animal: it says it cannot go out wearing one. Rashi explains "משום שמא תפול ומייתי לה הוא", "because it may fall and he will bring it".
- On 54 amud 2, the Talmud is discussing a neck cradle, used to prevent an animal from gnawing at its skin: it says the animal cannot go out wearing it. Rashi explains "וחשובים הן… ואי נפלו חייס עלייהו ומייתי להו בשגגה", "they're valuable… and if they fall he will have pity on them and bring them by accident".
Why specifically and only in the latter case does Rashi say "בשגגה", "by accident"?