SAN FRANCISCO — There isn’t much that manager Bob Melvin hasn’t seen on a baseball field. Friday night provided him — and most of his players — with a first.

In the top of the fourth inning, Sean Murphy hit a bases loaded line drive at right fielder Mike Yastrzemski. Yastrzemski made the catch and fired home; Olson tagged up and burst for the plate. As Yastrzemski’s throw arrived and Olson slid home safe, a second ball entered the playing field.

From the angle of the throw, the ball appeared to be thrown by a fan from the upper deck. The throw did not interfere with the play, but the umpiring crew convened regardless. The play stood, giving Atlanta its first run of the game.

“That’s a first,” Melvin said after the Giants’ 5-4 win over the Braves. “I’ve never seen that before. I don’t know where it came from. I don’t know what the rule is. I think everybody was a little surprised by it. It didn’t affect the play, but it was extremely odd to see another baseball come onto the field.”

Hayden Birdsong, the pitcher at the time of the incident, didn’t see the throw come down from the stands.

“I thought it was the ball that was thrown home,” Birdsong said. “I was like, ‘Why is the ball way out there?’ Then I realized Patty had the other ball. I was like, ‘What is going on?’ I had no idea.”

Birdsong, 23, said he had never seen anything like this before.

Color commenter Mike Krukow, calling the game alongside long-time broadcast partner Duane Kuiper, echoed Birdsong and Melvin’s sentiments.

“Came out of the upper deck,” Krukow said on the television broadcast. Some clown threw the baseball onto the field. I have never seen that.”