Round one of the Roldan fight, yeah. As Asterion says, it's highly debatable as to whether it should have been called a real knockdown. I've seen worse knockdown calls, but Hagler slipped momentarily, Roldan swung a wild right around the back of Hagler's head, sort of pulled him forward, and Hagler fell to his hands and knees for just a moment. In no way was he even close to hurt.
For all intents and purposes, absolutely not. Roldan shoved Hagler down with his arm on the back of Hagler's neck, and referee Carlos Padilla blew the call, simple as that. I remember that at the press conference following Hagler's win, that Marv was going to petition to have Padilla's errant ruling overturned, especially since it wasn't a scoring blow to a permitted area of Hagler's body.
Like Michael Spinks before he moved up to Heavyweight (i.e Dwight Qawi), got fluked by Juan Roldan, not a true knockdown.
Hagler was a tough sob. He would just stand in the pocket and hit his opponent until something broke =] The Roldan "knockdown" was also bull****.
There was a story doing the rounds a few years ago that MMH was floored in his fight with the Argentine Norberto Cabrera in Monte Carlo in '79. Not sure whether this is an urban myth? There seemed to be a lot of mystery surrounding this one...I'm not sure if the video is available of this fight...anybody know the truth behind this one?
Though Hagler was extremely sensitive about the topic, his skull was abnormally thick. He was only down once in his career.