I have an issue with iCal regarding the birthday calendar.
It works pretty fine syncing with address book, but it ads the birthday obviously twice. The birthday itself and the day before the birthday. When refering the URL in iCal it directs me to the Contact card of the person and there is only the correct date.
Why does iCal do that? And more important, how can I solve that problem and remove all the wrong dates? Because it is anoying to have birthday reminders for people twice. It also looks pretty dumm in the notification center of Mountain Lion. With two birthdays of the same person in a row. Mostly it has one year difference as well, so the same person turns lets say 29 the one day and 30 the other.
Problem seems to be solved now in 10.8.2!