Let's get one thing out of the way first—Barcelona's defense was solid on the night.
Everyone did their job. The full-backs were key in the attack, the central defenders held Sociedad tight, Busquets dispossessed early.
But that doesn't mean there were not signs of weakness. At least three defenders provided reason to worry.
Dani Alves was great in attack and had little to do defensively, but what was he doing on the lone goal allowed?
The right-back got caught far out of position when Illarramendi gave the night's best pass before Castro made Alves pay for his mistake.
It was just one lapse, but imagine if Real had more possession on the evening. Imagine if that had been Cristiano Ronaldo instead.
At the other end, Jordi Alba was often caught too far forward and he will need to rein in his attacking tendencies a bit.
It was great seeing them take part in the attack, but having both full-backs so far forward left huge amounts of space out wide.
At times, there was just one defender back and Busquets shielding another defensive midfielder (Mascherano) against the counterattack.
In the middle, Javier Mascherano didn't have much to do, but he was caught out of position at least twice. One of those times he was forced to foul just to prevent a goal-scoring opportunity.
Last season we saw how poor Masch's positioning can be and how vulnerable he makes the team on the counter and against crosses.
Overall, the mistakes were minimal and the overall performances of all four defenders was great, but there were enough mistakes to at least need a closer look going forward.