Safety’s big night marred by mistakes.

Safety Justin Reid has prevented numerous touchdowns for Stanford this year, but even great players sometimes have off nights.

“I don’t think I played up to expectations,” Reid said Friday night.

He had a fumble recovery and nine tackles in the Pac-12 title game against USC at Levi’s Stadium, second to Harrison Phillips’ 13 on the Cardinal side. But he misfired on a few big plays, including a pass interference infraction, one of three Stanford penalties that aided USC’s march to a field goal in the second quarter in the Trojans’ 31-28 win.

When Sam Darnold launched a 49-yard pass to Steven Mitchell in the third quarter, Reid misjudged the ball and missed it with a desperate leap.

“You’ve got to give them credit,” Reid said. “He’s got a great arm and made a good throw. But at the end of the day, that ball’s on me.”

The play put the ball on the 1-yard line, and USC quickly scored. Reid also was beaten by Michael Pittman on a 54-yard pass that set up the eventual decisive touchdown. Dueling fumbles: USC’s Stephen Carr fumbled on the first play of a third-quarter drive when he was hit by Alameen Murphy. Reid recovered at the Trojans’ 45.

But right away the Cardinal fumbled, too. K.J. Costello tried to hit Connor Wedington, and Ajene Harris knocked it down. But the throw was ruled a backward pass, making it a live ball. While Harris celebrated, Wedington picked up the ball.

Stanford challenged the backward-pass call but the ruling was upheld, costing the Cardinal a timeout. The Cardinal were soon forced to punt as Harris nearly intercepted a 3rd-and-23 pass from Costello. Alfieri plays: Linebacker Joey Alfieri played for Stanford after being questionable this week with a shoulder injury, but linebacker Curtis Robinson didn’t play. Defensive end Eric Cotton had been ruled out early in the week. … Jake Bailey gave another strong performance for the Cardinal, averaing 45.6 on his five punts and kicking four touchbacks on as many kickoffs. Helton pushes for playoff spot: USC head coach Clay Helton said he thinks the Trojans may deserve a spot in the College Football Playoff despite their two losses.

“An 11-win football team that had a great strength of schedule,” he said. “All of a sudden, if you have four two-loss champions, I think we deserve to be talked about.”