I've seen those replays over and over. When I posted about the "knees" comment I heard, it was that Barbaro had fallen to his knees at some point, but I do think he was mounted at the time and it was possibly pre-Derby.
It was just race people talking and someone said they had seen it happen - but I got the impression it wasn't yesterday.
If I run across the comment again, I'll post it here. It wasn't about his knees hitting the gate, I know that. All those same people said he didn't - but I do know it's still a possibility, too.
I think you are right that he stumbled coming out of the gate at the Derby... or one of them did.
Jockey Edgar Prado, who won his first Kentucky Derby victory in seven tries, said that he had smooth sailing after Barbaro made a slight misstep at the start.
“He stumbled a little bit coming out of the gate but he recovered right away,” said Prado. “You see what he did when I turned him loose, he took off like a rocket. I never had a doubt in this horse....
Source: Kentucky Derby News 05.06.2006a