Real Madrid arrived to a hostile reception from Racing fans at the Santander Airport ahead of Wednesday's match. Much of the abuse was, of course, aimed at Cristiano Ronaldo, who seemed to react to their words by flashing his middle finger their way before boarding the team coach. You can see him do it in the above video (he's in the black cap).
According to Marca, Ronaldo later claimed that his gesture wasn't intended for the fans, and was actually aimed at short-tempered teammate Pepe. At which point everyone probably responded, "Oh, well OK then."
Ronaldo does say it was only a joke. Even though he was being berated at the time and didn't appear to be in the most joyous of moods.