Reference for Bava Kamma 54:5
אמר רב זביד משמיה דרבא הוא הדין דאפי' שבר והאי דקתני נתקל איידי דבעי למתני סיפא ואם הוזק בה בעל חבית חייב בנזקו דדוקא נתקל אבל שבר לא מאי טעמא הוא דאזיק אנפשיה קתני רישא נתקל
R. Papa said: Our Mishnah is not consistent unless in accordance with Samuel or R. Johanan, for according to Rab why exemption only in the case of stumbling [over the pitcher]? Why not the same ruling even when one directly broke it? — R. Zebid thereupon said in the name of Raba: The same law applies even when the defendant directly broke it; for AND STUMBLES was inserted merely because of the subsequent clause which reads, IF THE OTHER ONE WAS INJURED BY IT, THE OWNER OF THE BARREL IS LIABLE TO COMPENSATE FOR THE DAMAGE; and which of course applies only to 'stumbling' but not to direct breaking, in which case it only stands to reason that it is the plaintiff who is to blame for the damage he caused to himself. It was therefore on this account that 'stumbling' was inserted in the commencing clause.