To reproduce:
- Set the value to February 29.
- Open the drop down and zoom 3 times.

Workaround:
private void Calendar_ZoomChanging(object sender, CalendarZoomChangingEventArgs e)
{
    RadCalendar calendar = sender as RadCalendar;
    if (calendar.FocusedDate.Month == 2 && calendar.FocusedDate.Day == 29)
    {
        calendar.FocusedDate = new DateTime(calendar.FocusedDate.Year, calendar.FocusedDate.Month, calendar.FocusedDate.Day - 1);
    }
}