When Peter goes to answer his iPhone, throughout the film you constantly hear two sounds when he answers it - the ringtone and the iPhone's "lock" sound. Normally, the iPhone only ever plays that sound when unlocking it. When you receive and answer a call, no sound is played other then the ringtone.
When Sydney is hit by the golf ball, the wound starts on his knee and is clearly shown, but later that day when he calls Pete about the Rush concert, the ice pack is now down near his foot, with no sign of the injury on his knee.
When Peter is staring at Sydney's card in his office booth, about to call him for the first time, the hand holding the card is not Paul Rudd's (cf. close up to Peter's hand when he originally receives Sydney's card).