The Vinegar Riverbank Incident is a minor feat of All Might, in which he saved a boy from drowning in vinegar.
The incident happened several years ago, before Sir Nighteye team up with All Might. A middle schooler fell into a river and began to drown. The boy had a Quirk that allows him to change the properties and quality of water, and in a panic, he activated his Quirk unconsciously, turning the water in the area of the river where he was drowning into vinegar.
Hearing his cries for help, All Might arrived and jumped into the river, rescuing the boy and saving the day. However, he suffered eye irritation as a result of the vinegar getting into his eyes. During the interview that followed, his eyes were all puckered up as he smiled. The boy he had saved apologized for the problems caused and thanked him for saving his life, but All Might gave him a witty response that it is he who should be grateful, because thanks to him, his skin feels ten years younger.
Since there was no villain involved, and the whole thing was kind of plain, the incident is usually overshadowed by his other exploits, so it is rarely talked about on fan sites, although Izuku Midoriya loves it, and the face that All Might had after the rescue is the one he usually imitates.
- The Vinegar Riverbank incident is only mentioned in the conversation between Izuku and Sir Nighteye in the manga. The anime shows the event.