ADDITIONAL SYSTEM INFORMATION :
The bug is ArrayList which is System / OS/ JRI independent, and is present in the master branch of the github openjdk repo clone as of Jan 25th 2021.

A DESCRIPTION OF THE PROBLEM :
In exotic circumstances (not involving threading, see code samples below), for ArrayList: You can make an iterator, then modify the list (and not through the iterator), then iterate on the iterator, and __no__ ConcurrentModificationException occurs. This is a bug: The javadoc of ArrayList explicitly promises that it always will.

This occurs because the hasNext method fails to check modCount; instead, it just checks if the iterator's 'position' field is equal to the backing list's 'size' field, and if so, returns false (thus e.g. ending a for(:) construct). That means that for example looping through an arraylist and removing the second-to-last element on its iterator results in the while loop ending as normal instead of the expected and according to javadoc spec required ConcurrentModificationException.

See lines 75-85 for the javadoc, I will quote here:

<p id="fail-fast">
 * The iterators returned by this class's {@link #iterator() iterator} and
 * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:
 * if the list is structurally modified at any time after the iterator is
 * created, in any way except through the iterator's own
 * {@link ListIterator#remove() remove} or
 * {@link ListIterator#add(Object) add} methods, the iterator will throw a
 * {@link ConcurrentModificationException}. Thus, in the face of
 * concurrent modification, the iterator fails quickly and cleanly, rather
 * than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.

which indicates that throwing that ConcurrentModificationException is __required__. The offending line is line 964, reproducing here:

public boolean hasNext() {
    return cursor != size;
}

This line should first invoke `checkForComodification();` prior to running its one line.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run this code:

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
for (String item : list) if ("1".equals(item)) list.remove("1");

List<String> list2 = new ArrayList<String>();
list2.add("1");
list2.add("2");
for (String item : list) if ("2".equals(item)) list.remove("2");


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The first block (the first for loop) should have thrown a ConcurrentModificationException. The javadoc of ArrayList itself demands that it do so.

ACTUAL -
No ConcurrentModificationException is thrown for the first block. As an example, the second block does indeed throw it, which highlights that you need a fairly specific scenario in order for the CoModEx to fail to be thrown.

---------- BEGIN SOURCE ----------
import java.util.*;

class OopsieArrayListBroken {
    public static void main(String[] args) {
        try {
            List<String> list = new ArrayList<String>();
            list.add("1");
            list.add("2");

            for (String item : list) {
                if ("1".equals(item)) {
                    list.remove(item);
                }
            }

            throw new AssertionError("We should not get here at all!");
        } catch (ConcurrentModificationException e) {
            // this is expected.
        }
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Well, I doubt any code is out there that relies on ConcurrentModificationException being thrown to work correctly; this is more a case where a bug is written in someone's java code, where this openjdk bug causes their bug to be harder to find.

FREQUENCY : always