Spot the ConcurrentModification Bug

ServiceNow's ticket queue has a small cleanup routine that removes a closed ticket ID from an in-memory list while walking it. QA flagged that it throws under certain conditions during a test run.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class TicketQueueCleaner {

    public static void main(String[] args) {
        List ticketIds = new ArrayList();
        ticketIds.add(101);
        ticketIds.add(102);
        ticketIds.add(103);
        ticketIds.add(104);

        Iterator iterator = ticketIds.iterator();
        while (iterator.hasNext()) {
            Integer id = (Integer) iterator.next();
            if (id.equals(102)) {
                ticketIds.remove(id);
            }
        }

        System.out.println("Remaining tickets: " + ticketIds);
    }
}

Explain exactly what's wrong with this code and what behavior it causes when it runs.