ConcurrentModificationException: Thrown when an object is concurrently modified while iterating through it, in a way that is not allowed.
Example:

import java.util.ArrayList;
import java.util.Iterator;
import java.util.ConcurrentModificationException;
ArrayList list = new ArrayList<>();
list.add("One");
list.add("Two");
list.add("Three");
Iterator iterator = list.iterator();
try {
 list.remove("Two");
 iterator.next();
} catch (ConcurrentModificationException e) {
 System.out.println("Caught ConcurrentModificationException: " + e.getMessage());
}