Plenty of rowdy music fans were around, but Johnson didn't expect any kind of violence. That's when the police charged.
"I was eating a hot dog; five minutes later I was wearing it," said Johnson, a freelance writer from Santa Cruz, Calif.
What Johnson and most of the others at the intersection didn't know was that the L.A.P.D. was under orders to secure the area before Democratic delegates began exiting the Staples Center a block away. The time was 11:30 p.m. Eastern Time. Police had about 20 minutes to, in their words, clear the area in order to protect delegates from any harassment.
Making the police even more nervous were a few music fans lobbing projectiles over a nearby parking lot fence at their ranks.
In the end, the mustard stains on her tank top were the least of Johnson's worries. As she sought refuge from the charging phalanx, she lost her step on a stairway, fell down, and starting taking blows from a police officer's billy club.
"There was no warning," Johnson said. "They are supposed to call for the crowd to disperse with a bullhorn."
At the time of the charge, Johnson said she was more interested in recovering her camera bag, which had fallen to the ground, than in trying to determine the identity of the police officer who beat her. She said she is considering filing a complaint with the American Civil Liberties Union.
According to witnesses who were in the area when police made their charge, others suffered more severe beatings that Johnson did. But police reported no serious injuries, and in the minutes after the charge, the area cleared quickly.
Tuesday, likely the convention's quietest day, may provide relief for police and protesters both. But overall, the sentiment on the street was that the Los Angeles Police Department overreacted to what amounted to a peaceful, if profane and slightly raucous, gathering of activists - and, in larger numbers, music fans.
The L.A.P.D., which had kept a high profile on downtown streets throughout the day, defended its actions - and vowed that any further trouble would be met with an equally strong response.
"Today, tomorrow or the next day, or the next day, or the next week, our response will be exactly the same," said Cmdr. David Kalish, a police spokesman.