First Half
27': Van Persie was seen clipping Messi's ankle in a late challenge. The referee correctly called the foul, but did not issue a yellow card to Van Persie. I think one could argue that Van Persie made a reckless challenge by clipping Messi late. I had a hard time deciding if I should even add this to my review.
28': Sagna gets yellow card for time wasting by throwing the ball to a teammate after being called for a foul. The referee proceeds to book Sagna. I didn't think it was worth a yellow card to be honest. It didn't end up to make a difference at the end of the match.
32': Messi was swept inside the penalty box. A clear penalty not given by the referee.
44': Abidal holds Van Persie's neck during a confrontation between both teams. No card was given but one could argue a case for a yellow card on Abidal. Again, Abidal didn't receive a yellow card later in the match so it wouldn't have made a difference.
45': Van Persie blatantly shoves Alves in the face off the ball. The referee gives Van Persie a yellow card. One could argue a case for a straight red card in this instance. Van Persie had his back to the ball and just blatantly shoved Alves' face. One could also argue Van Persie should have had his second yellow card of the night if they believe he deserved a yellow card in the 27'.
Second Half
55': Van Persie shoots the ball after the whistle and gets his second yellow. A very harsh call. No card was warranted.
70': Koscielny takes down Pedro in the box and a penalty is given. However, no card was given out for the Arsenal defender. I believe a card should have been given for the challenge. What card? One could argue a straight red card. It was considered as denying an obvious goal scoring opportunity. One could argue it was only deserving of a yellow card. Either way, that would have been Koscielny's second yellow of the night and he would be sent off with 20 minutes to go (even more if Messi had missed the penalty and it went into overtime).
Summary
Despite all the talk of bad officiating, I think the officiating wasn't as bad as people think. The assistant referees made no wrong calls throughout the match, which is quite impressive. The center referee made two big wrong calls. The first one was the missed penalty call for Barcelona in the 32'. The other big call is the second yellow card for Van Persie. In fact it seemed the referee was too happy to hand out cards for time wasting when they were not warranted. However, I think Arsenal were perhaps lucky not to see Van Persie sent off earlier than the 55'. Now I could continue to talk about how Arsenal did not deserve to go through, but that is not the purpose of this thread. I just wanted to comment on the referees' performances and show that bad calls were made for both teams.