ArrayList Modification During Iteration

What is the primary issue that arises when the provided code is executed?

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

public class IteratorTest {
    static ArrayList<String> list = new ArrayList<>();
    public static void main(String[] args) {
        list.add("A");
        list.add("B");
        list.add("C");
        Thread t = new Thread(() -> {
            synchronized(list) {
                Iterator<String> it = list.iterator();
                while (it.hasNext()) {
                    String s = it.next();
                    if (s.equals("B")) {
                        list.remove(s);
                    }
                }
            }
        });
        t.start();
    }
}