It's not often you see fans cheering on the same team get in a nasty brawl -- but that's exactly what happened in San Francisco -- with haymakers flying ... and even a knockout!!!
Unclear what led to the massive melee Saturday night at Levis Stadium as the 49ers hosted the Denver Broncos in a preseason matchup, but it got messy fast. You can see lots of pushing, shouting and confusion before blows are exchanged.
At one point, a guy in a Jerry Rice jersey starts absolutely unleashing some fists on a dude in a white t-shirt, connecting several times with his face.