How does the Collections Framework's fail-fast modCount mechanism actually work internally, and why is it described as "best-effort" rather than a strict guarantee?

Each structurally-modifying operation (add, remove, but not set()) on a collection like ArrayList increments an internal modCount field; an Iterator captures the modCount value at its creation and compares it against the collection's current modCount on every next() call, throwing ConcurrentModificationException if they've diverged -- this is explicitly documented as "best-effort" because it's fundamentally a heuristic bug-detection aid, not a formal correctness guarantee: it cannot detect every possible concurrent modification scenario (a modification that happens to leave modCount unchanged by coincidence, or certain modification patterns during multi-threaded access, could theoretically slip through undetected), so code should never rely on ConcurrentModificationException as a correctness mechanism, only as a debugging aid for catching common single-threaded misuse.

// Simplified conceptual illustration of ArrayList's internal fail-fast check
class SimplifiedIterator {
    int expectedModCount = modCount;  // captured at iterator creation

    Object next() {
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        // ... return next element
    }
}