With Cal being the only PAC-12 team undefeated after this weekend, I was wondering when this last happened.

The last time this might've happened that I found was 1977. It was just the PAC-8 then, and Cal started off the season 4-0, as did USC. On October 8, both Cal and USC lost. Depending on the order (i can't find kick-off times), Cal may have been briefly the only undefeated team.

Before that, and the last time I can find that they ended the weekend the only undefeated team inn the conference, was 1950. Predating the PAC entirely, this was part of the PCC, a time where Idaho was in the same conference as the teams that would become the PAC-8. That year, Cal won their first 9 games, until a tie with Stanford on November 25 (Washington and Stanford were the last two other teams to lose, both on October 21).

That season, Cal wouldn't actually lose a game until January 1 of 1951, when they would lose to Michigan in the Rose Bowl 14-6. They finished the season ranked 4th in the coaches' poll and 5th in the AP poll.

(Hopefully I didn't miss something, this was done by hand)