As people were enjoying the game, police say a San Bruno man threw an M-80 explosive into the bleacher section during the seventh inning of the game against the Dodgers.
An M-80 firecracker is illegal to own or possess, and certainly a crime to light in a crowd. It's been used to blow up toilets and has been known to take off fingers or your hand.
During the Giants game, at around 9:30 p.m., someone threw an M-80 firecracker into a crowd of baseball fans.
Talina Rocha was just two seats away.
"There was a huge pop right next to us and white smoke was everywhere and people were ducking," she said.
She tweeted a picture of police responding to the Coors Light deck.
"A lot of people said their ears started hurting because they were so close," Rocha said. "One man, the smoke got in his eyes."
Other fans in the stadium heard the blast but didn't understand what happened.
"Some music went off and then it went boom, and then the whole right field just started staring up," one fan said.
Another fan added, "We thought it was just a fake firework, a firework that went off accidentally, we saw some smoke by the outfield, that's pretty much all it was."
On Thursday, as fans flooded into the stadium and went through security, many were unaware of the M-80 explosion during the game the night before.
"Um, I don't even know what to say," said one fan.
Police arrested 21-year-old Paolo Pavone of San Bruno. He is in jail on $2.1 million dollars bond.
"We believe it was an isolated incident," said Officer Gordon Shyy. "We're lucky that no one was seriously injured, we're also happy that we were able to identify him and arrest him on the spot."
While Rocha is thankful the suspect is in jail, she's concerned about what the M-80 means for future fans' safety.
"It was scary," she said. "It was scary that it got through security."
The San Francisco Giants wouldn't go on camera because this is an ongoing police investigation, but they told us this is the first incident of its kind in their 15 years at AT&T Park.