Too many mental errors.

Lincoln Riley talked earlier this week about what he saw after diving into the Stanford tape, and he shared with reporters that he still sees guys not fully trusting their stuff.

“I’ve had players that have been in our systems for multiple years, and it’s still not 100% trust,” Riley said. “You get there on Saturdays and you find out how ironclad that trust is.”

Riley talked a ton about that 100% trust, how the USC coaching staff needs it from the players and the players need it back from the staff. It takes time to get there, but Riley said he’s not totally sure anyone — even the guys who followed him from Oklahoma to USC — has it yet.

“It’s hard to get to that point. That’s something you’ve got to really strive towards. But (quarterback Caleb Williams) and I both are trying to strive to get there,” Riley said. “When guys trust what we’re doing and we don’t make many mental errors, we do some pretty good things here through two games. When we don’t, we don’t.

“As coaches, we’ve got to trust them too. It’s a two-way street. We’re building our trust in them. They’re building their trust in us. We told the guys, the staff, from Day 1, we’re not going to use the ‘This is Year 1’ crutch. This is all we got right now. We got to make the best of it that we possibly can and we can definitely get a lot better.”

The Trojans roared to a 35-14 first-half lead against Stanford. Williams led touchdown drives on the first five possessions. But after the team came out of the locker room, they put just two field goals on the board. After the game, “complacency” was a word that was tossed around.

“At times, we intentionally slowed it down a little bit,” Riley said, “but slowing it down doesn’t mean you don’t execute. You can call a play and snap it with 30 seconds left or snap with five seconds left, you’ve still got to execute. And the reality is when we slowed it down, our execution went down.”

Riley specifically called out a pair of third-quarter penalties. One of them was a holding call on tackle Courtland Ford  and another an offensive pass interference call on wideout Brenden Rice. Both negated what would have been third-down conversions. Riley also expressed some frustration with a few blitz pickups that were missed and results in hits on the quarterback.

“We did things in the second half that bad offenses do, and when you do that against good football teams, you’re not gonna play the way you want to,” he said. “Again, whether it’s complacency, lack of focus, lack of trust, whatever it is, it’s got to get better.”

The seventh-ranked Trojans (2-0) will look for a more complete performance against Fresno State (1-1) on Saturday (7:30 p.m. PT, FOX).